course MTH 272 02/02, 0420 assignment #001001. `query 1
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20:18:50 INTRODUCTORY NOTE: The typical student starting out a second-semester calculus course it typically a bit rusty. It is also common that students you tend to use the calculator in appropriately, replacing analysis with calculator output. The calculator is in this course to be used to enhance the analysis but not to replace it, as you will learn on the first assignments. Some first-semester courses emphasize calculator over analysis rather than calculator as an adjunct to analysis, and even when that is not the emphasis the calculator tricks are all some students com away with. A student who has completed a first-semester course has the ability to do this work, but will often need a good review. If this is your case you will need to relearn the analytical techniques, which you can do as you go through this chapter. A solid review then will allow you to move along nicely when we get to the chapters on integration, starting with Ch 5. Calculator skills will be useful to illuminate the analytical process throughout. THis course certainly doesn't discourage use of the calculator, but only as an adjunct to the analytical process than a replacement for it. You will see what that means as you work through Chapter 4. If it turns out that you have inordinate difficulties with the basic first-semester techniques used in this chapter, a review might be appropriate. I'll advise you on that as we go through the chapter. For students who find that they are very rusty on their first-semester skills I recommend (but certainly don't require) that they download the programs q_a_cal1_1_13... and q_a_cal1_14_16... , from the Supervised Study Current Semester pages (Course Documents > Downloads > Calculus I or Applied Calculus I) and work through all 16 assignments, with the possible exception of #10 (a great application of exponential functions so do it if you have time), skipping anything they find trivial and using their own judgement on whether or not to self-critique. The review takes some time but will I believe save many students time in the long run. For students who whoose to do so I'll be glad to look at the SEND files and answer any questions you might have. Please take a minute to give me your own assessment of the status of your first-semeseter skills.
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RESPONSE --> I believe that my first-semester skills are relatively solid. I do plan, however, to review my notes and homework for the first semester and work through the suggested assignments as needed. I think a quick yet focused review should help me brush away any existing cobwebs.
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20:18:54 INTRODUCTORY NOTE: The typical student starting out a second-semester calculus course it typically a bit rusty. It is also common that students you tend to use the calculator in appropriately, replacing analysis with calculator output. The calculator is in this course to be used to enhance the analysis but not to replace it, as you will learn on the first assignments. Some first-semester courses emphasize calculator over analysis rather than calculator as an adjunct to analysis, and even when that is not the emphasis the calculator tricks are all some students com away with. A student who has completed a first-semester course has the ability to do this work, but will often need a good review. If this is your case you will need to relearn the analytical techniques, which you can do as you go through this chapter. A solid review then will allow you to move along nicely when we get to the chapters on integration, starting with Ch 5. Calculator skills will be useful to illuminate the analytical process throughout. THis course certainly doesn't discourage use of the calculator, but only as an adjunct to the analytical process than a replacement for it. You will see what that means as you work through Chapter 4. If it turns out that you have inordinate difficulties with the basic first-semester techniques used in this chapter, a review might be appropriate. I'll advise you on that as we go through the chapter. For students who find that they are very rusty on their first-semester skills I recommend (but certainly don't require) that they download the programs q_a_cal1_1_13... and q_a_cal1_14_16... , from the Supervised Study Current Semester pages (Course Documents > Downloads > Calculus I or Applied Calculus I) and work through all 16 assignments, with the possible exception of #10 (a great application of exponential functions so do it if you have time), skipping anything they find trivial and using their own judgement on whether or not to self-critique. The review takes some time but will I believe save many students time in the long run. For students who whoose to do so I'll be glad to look at the SEND files and answer any questions you might have. Please take a minute to give me your own assessment of the status of your first-semeseter skills.
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RESPONSE -->
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20:20:51 You should understand the basic ideas, which include but are not limited to the following: rules of differentiation including product, quotient and chain rules, the use of first-derivative tests to find relative maxima and minima, the use of second-derivative tests to do the same, interpreation of the derivative, implicit differentiation and the complete analysis of graphs by analytically finding zeros, intervals on which the function is positive and negative, intervals on which the function is increasing or decreasing and intervals on which concavity is upward and downward. Comment once more on your level of preparedness for this course.
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RESPONSE --> Having performed well in the first-semester of this course, I feel that a quick and focused review help refresh my memory of these particular subjects and procedures.
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20:20:54 You should understand the basic ideas, which include but are not limited to the following: rules of differentiation including product, quotient and chain rules, the use of first-derivative tests to find relative maxima and minima, the use of second-derivative tests to do the same, interpreation of the derivative, implicit differentiation and the complete analysis of graphs by analytically finding zeros, intervals on which the function is positive and negative, intervals on which the function is increasing or decreasing and intervals on which concavity is upward and downward. Comment once more on your level of preparedness for this course.
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RESPONSE -->
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20:24:42 4.1.16 (was 4.1.14): Solve for x the equation 4^2=(x+2)^2
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RESPONSE --> 4^2=(x + 2)^2 +-4= x + 2 4=x + 2 x=2 -4=x + 2 -6=x confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:24:50
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RESPONSE --> confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:27:32 The steps in the solution: 4^2 = (x+2)^2. The solution of a^2 = b is a = +- sqrt(b). So we have x+2 = +- sqrt(4^2) or x+2 = +- 4. This gives us two equations, one for the + and one for the -: x+2 = 4 has solution x = 2 x+2 = -4 has solution x = -6. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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20:33:56 4.1.28 (was 4.1.32) graph 4^(-x). Describe your graph by telling where it is increasing, where it is decreasing, where it is concave up, where it is concave down, and what if any lines it has as asymptotes.
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RESPONSE --> The graph of 4^(-x) is decreasing from left to right and is concave upward. It crosses the y-axis at (0,1). The horizontal asymptote is the x-axis. confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:34:15 Many students graph this equation by plugging in numbers. That is a start, but you can only plug in so many numbers. In any case plugging in numbers is not a calculus-level skill. It is necessary to to reason out and include detailed reasons for the behavior, based ultimately on knowledge of derivatives and the related behavior of functions. A documented description of this graph will give a description and will explain the reasons for the major characteristics of the graph. The function y = 4^-x = 1 / 4^x has the following important characteristics: For increasing positive x the denominator increases very rapidly, resulting in a y value rapidly approaching zero. For x = 0 we have y = 1 / 4^0 = 1. For decreasing negative values of x the values of the function increase very rapidly. For example for x = -5 we get y = 1 / 4^-5 = 1 / (1/4^5) = 4^5 = 1024. Decreasing x by 1 to x = -6 we get 1 / 4^-6 = 4096. The values of y more and more rapidly approach infinity as x continues to decrease. This results in a graph which for increasing x decreases at a decreasing rate, passing through the y axis at (0, 1) and asymptotic to the positive x axis. The graph is decreasing and concave up. When we develop formulas for the derivatives of exponential functions we will be able to see that the derivative of this function is always negative and increasing toward 0, which will further explain many of the characteristics of the graph. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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20:43:01 How does this graph compare to that of 5^-x, and why does it compare as it does?
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RESPONSE --> The graph of 5^(-x) is also decreasing from left to right and is concave upward. The graph however is closer to the y-axis, making it appear steeper. This graph also passes through (0,1) and has a horizontal asymptote at the x-axis. The difference in these graphs falls on the negative side of the x-axis. As part of the exponential equation, we are taking the opposite of the x-value as our exponent. So, 4^(-(-2)) equals 4^2 equals 16 and 5^(-(-2)) equals 5^2 equals 25. Since the 4 and 5 of these equations are being raised to the same exponential value at each x-value, the steepness of the 5^-x graph is a result of a larger base number. confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:43:10 the graphs meet at the y axis; to the left of the y axis the graph of y = 5^-x is higher than that of y = 4^-x and to the right it is lower. This is because a higher positive power of a larger number will be larger, but applying a negative exponent will give a smaller results for the larger number. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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20:45:19 4.2.20 (was 4.1 #40) graph e^(2x) Describe your graph by telling where it is increasing, where it is decreasing, where it is concave up, where it is concave down, and what if any lines it has as asymptotes.
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RESPONSE --> The graph of e^(2x) is increasing from left to right (at an increasing rate) and is concave upward. It crosses the y-axis at (0,1) and has a horizontal asymptote at the x-axis. confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:45:32 For large numbers x you have e raised to a large power, which gets extremely large. At x = 0 we have y = e^0 = 1. For large negative numbers e is raised to a large negative power, and since e^-a = 1 / e^a, the values of the function approach zero. } Thus the graph approaches the negative x axis as an asymptote and grows beyond all bounds as x gets large, passing thru the y axis as (0, 1). Since every time x increases by 1 the value of the function increases by factor e, becoming almost 3 times as great, the function will increase at a rapidly increasing rate. This will make the graph concave up. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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20:47:43 The entire description given above would apply to both e^x and e^(2x). So what are the differences between the graphs of these functions?
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RESPONSE --> The e^x graph is increasing at a slower rate than the e^2x graph, i.e., it is further from the y-axis and is less steep. This difference is because the 2 in the e^2x equation makes the exponent to which e is raised twice the value of the exponent in the e^x graph. confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:55:11 How did you obtain your graph, and what reasoning convinces you that the graph is as you described it? What happens to the value of the function as x increases into very large numbers? What is the limiting value of the function as x approaches infinity?
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RESPONSE --> One way to obtain the graph is by entering the provided equations into a graphing calculator and noting the differences between the two results. Another way to obtain the graph, or to verify the results obtained by calculator, would be to make a chart for each equation, e^x and e^2x, and then determine the value of each for several different x-values. When we make this table, we are able to find that the resulting value of the functions, or y, are greater numbers for e^2x. As the x-values increase, the value of the function continues to increase. There is no limiting value of the function as x nears infinity. confidence rating #$&*: 1 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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20:55:26 *& These questions are answered in the solutions given above. From those solutions you will ideally have been able to answer this question. *&*&
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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21:04:40 4.2.32 (formerly 4.2.43) (was 4.1 #48) $2500 at 5% for 40 years, 1, 2, 4, 12, 365 compoundings and continuous compounding
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RESPONSE --> A=P( 1 + (r/n))^(nt) P=2500 r=5%=.05 t=40yrs n=1 A=2500(1 + (.05/1))^(1*40)=$17,599.97 n=2 A=2500(1 + (.05/2))^(2*40)=$18,023.92 n=4 A=2500(1 + (.05/4))^(4*40)=$18,245.05 n=12 A=2500(1 + (.05/12))^(12*4)=$18,396.04 n=365 A=2500(1 + (.05/365))^(365*40)=$18,470.11 Continuous Compounding: A=Pe^(rt) A=2500 e^(.05)(40)=$18,472.64 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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21:04:52 A = P[1 + (r/n)]^nt A = 2500[1 + (0.05/1]^(1)(40) = 17599.97 A = 2500[1 + (0.05/2]^(2)(40) = 18023.92 A = 2500[1 + (0.05/4]^(4)(40) = 18245.05 A = 2500[1 + (0.05/12]^(12)(40) = 18396.04 A = 2500[1 + (0.05/365]^(365)(40) = 18470.11
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RESPONSE --> confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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21:05:30 How did you obtain your result for continuous compounding?
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RESPONSE --> The results for continuous compounding are found using the equation: A=Pe^(rt). confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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21:05:44 For continuous compounding you have A = Pe^rt. For interest rate r = .05 and t = 40 years we have A = 2500e^(.05)(40). Evaluating we get A = 18472.64 The pattern of the results you obtained previously is to approach this value as a limit. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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21:11:46 4.2.40 (was 4.1 #60) typing rate N = 95 / (1 + 8.5 e^(-.12 t)) What is the limiting value of the typing rate?
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RESPONSE --> By graphing the function for typing rate, we can see that N begins to stabilize at approximately y=93. confidence rating #$&*: 1 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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21:12:08 As t increases e^(-.12 t) decreases exponentially, meaning that as an exponential function with a negative growth rate it approaches zero. The rate therefore approaches N = 95 / (1 + 8.5 * 0) = 95 / 1 = 95. *&*&
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RESPONSE --> Though my answer is near the given answer, I arrived at my response by observing the graph rather than calculating it mathematically.
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ow long did it take to average 70 words / minute?
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RESPONSE --> N = 95 / (1 + 8.5 e^(-.12 t)) 70 = 95 / (1 + 8.5 e^(-.12 t)) (1 + 8.5 e^(-.12 t))=95/70 1 + 8.5 e^(-.12 t)=1.357 8.5e^(-.12t)=.357 e^(-.12t)=.042 ln e^(-.12t)=ln(.042) -.12t=-3.17 t=26.4 confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:33:27 *& According to the graph of the calculator it takes about 26.4 weeks to get to 70 words per min. This result was requested from a calculator, but you should also understand the analytical techniques for obtaining this result. The calculator isn't the authority, except for basic arithmetic and evaluating functions, though it can be useful to confirm the results of actual analysis. You should also know how to solve the equation. We want N to be 70. So we get the equation 70=95 / (1+8.5e^(-0.12t)). Gotta isolate t. Note the division. You first multiply both sides by the denominator to get 95=70(1+8.5e^(-0.12t)). Distribute the multiplication: 95 = 70 + 595 e^(-.12 t). Subtract 70 and divide by 595: e^(-.12 t) = 25/595. Take the natural log of both sides: -.12 t = ln(25/595). Divide by .12: t = ln(25/595) / (-.12). Approximate using your calculator. t is around 26.4. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:33:32 How many words per minute were being typed after 10 weeks?
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RESPONSE --> N=95 / (1+8.5e^(-0.12t)) N=95 / (1+8.5e^(-0.12(10)) N=26.7 confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:33:38 *& According to the calculator 26.6 words per min was being typed after 10 weeks. Straightforward substitution confirms this result: N(10) = 95 / (1+8.5e^(-0.12* 10)) = 26.68 approx. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:33:43 Find the exact rate at which the model predicts words will be typed after 10 weeks (not time limit here).
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RESPONSE --> N=95 / (1+8.5e^(-0.12t)) N=95 / (1+8.5e^(-0.12(10)) N=26.7 confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:33:48 The rate is 26.6 words / minute, as you found before. Expanding a bit we can find the rate at which the number of words being typed will be changing at t = 10 weeks. This would require that you take the derivative of the function, obtaining dN / dt. This question provides a good example of an application of the Chain Rule, which might be useful for review: Recall that the derivative of e^t is d^t. N = 95 / (1 + 8.5 e^(-.12 t)), which is a composite of f(z) = 1/z with g(t) = (1 + 8.5 e^(-.12 t)). The derivative, by the Chain Rule, is N' = g'(t) * f'(g(t)) = (1 + 8.5 e^(-.12 t)) ' * (-1 / (1 + 8.5 e^(-.12 t))^2 ) = -.12 * 8.5 e^(-.12 t)) * (-1 / (1 + 8.5 e^(-.12 t))^2 ) = 1.02 / (1 + 8.5 e^(-.12 t))^2 ). **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:34:28 4.3.8 (was 4.2 #8) derivative of e^(1/x)
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RESPONSE --> e^(1/x) e^(x^-1) e^u(du/dx)=(e^(x^-1))(-1x^-2) (e^(1/x))(-1/x^2) confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:34:49 There are two ways to look at the function: This is a composite of f(z) = e^z with g(x) = 1/x. f'(z) = e^z, g'(x) = -1/x^2 so the derivative is g'(x) * f'(g(x)) = -1/x^2 e^(1/x). Alternatively, and equivalently, using the text's General Exponential Rule: You let u = 1/x du/dx = -1/x^2 f'(x) = e^u (du/dx) = e^(1/x) * -1 / x^2. dy/dx = -1 /x^2 e^(1/x) **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:36:23 Extra Question: What is the derivative of (e^-x + e^x)^3?
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RESPONSE --> (e^-x + e^x)^3 [3(e^-x + e^x)^2] * (e^x - e^-x) confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:36:41 This function is the composite f(z) = z^3 with g(x) = e^-x + e^x. f ' (z) = 3 z^2 and g ' (x) = - e^-x + e^x. The derivative is therefore (f(g(x)) ' = g ' (x) * f ' (g(x)) = (-e^-x + e^x) * 3 ( e^-x + e^x) ^ 2 = 3 (-e^-x + e^x) * ( e^-x + e^x) ^ 2 Alternative the General Power Rule is (u^n) ' = n u^(n-1) * du/dx. Letting u = e^-x + e^x and n = 3 we find that du/dx = -e^-x + e^x so that [ (e^-x + e^x)^3 ] ' = (u^3) ' = 3 u^2 du/dx = 3 (e^-x + e^x)^2 * (-e^-x + e^x), as before. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:36:57 4.3.22. What is the tangent line to e^(4x-2)^2 at (0, 1)?
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RESPONSE --> y=(e^(4x) - 2)^2 2(e^(4x) - 2) * (e^(4x))*4 At x=0 2(e^(4*0) - 2) * (e^(4*0))*4 2(1 - 2) * (1)(4) 2(-1)*4 -8=m y - 1 = -8(x - 0) y - 1 = -8x y=-8x + 1 confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:37:05 FIrst note that at x = 0 we have e^(4x-2) = e^(4*0 - 2)^2 - e^(-2)^2, which is not 1. So the graph does not pass through (0, 1). The textbook is apparently in error. We will continue with the process anyway and note where we differ from the text. }The function is the composite f(g(x)) wheren g(x) = e^(4x-2) and f(z) = z^2, with f ' (z) = 2 z. The derivative of e^(4x-2) itself requires the Chain Rule, and gives us 4 e^(4x-2). So our derivative is (f(g(x))' = g ' (x) * f ' (g(x)) = 4 (e^(4x-2) ) * 2 ( e^(4x - 2)) = 8 ( e^(4x - 2)). Now at x = 0 our derivative is 8 ( e^(4 * 0 - 2)) = 8 e^-2 = 1.08 (approx). If (0, 1) was a graph point the tangent line would be the line through (0, 1) with slope 1.08. This line has equation y - 1 = .0297 ( x - 0), or solving for y y = .0297 x + 1. As previously noted, however, (0, 1) is not a point of the original graph.
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RESPONSE --> I really struggled with this problem and decided to refer to the book to find an odd numbered problem to work through so I would be provided with the answer. I then discovered that the equation for 22 in the book (y=(e^(4x) -2)^2) didn't match the Query problem of e^(4x-2)^2. I'm not sure which is the correct notation, but I found it MUCH simpler to find the answer to the equation given in the book.
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11:37:12 4.3.26 (formerly 4.3.24) (was 4.2.22) implicitly find y' for e^(xy) + x^2 - y^2 = 0
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RESPONSE --> e^(xy) + x^2 - y^2 = 0 xe^(xy)(dy/dx) + (1)ye^(xy) + 2x - 2y(dy/dx)=0 x e^(xy)(dy/dx) - 2y(dy/dx) = -ye^(xy) - 2x (dy/dx)(xe^(xy) - 2y) = -ye^(xy) - 2x dy/dx = [-ye^(xy) - 2x] / [xe^(xy) - 2y] confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:37:23 The the q_a_ program for assts 14-16 in calculus 1, located on the Supervised Study ... pages under Course Documents, Calculus I, has an introduction to implicit differentiation. I recommend it if you didn't learn implicit differentiation in your first-semester course, or if you're rusty and can't follow the introduction in your text. The derivative of y^2 is 2 y y'. y is itself a function of x, and the derivative is with respect to x so the y' comes from the Chain Rule. the derivative of e^(xy) is (xy)' e^(xy). (xy)' is x' y + x y' = y + x y '. the equation is thus (y + x y' ) * e^(xy) + 2x - 2y y' = 0. Multiply out to get y e^(xy) + x y ' e^(xy) + 2x - 2 y y' = 0, then collect all y ' terms on the left-hand side: x y ' e^(xy) - 2 y y ' = -y e^(xy) - 2x. Factor to get (x e^(xy) - 2y ) y' = - y e^(xy) - 2x, then divide to get y' = [- y e^(xy) - 2x] / (x e^(xy) - 2y ) . **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:37:31 4.3.34 (formerly 4.3.32) (was 4.2 #30) extrema of x e^(-x)
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RESPONSE --> By analyzing the graph of the function x e^(-x), a maximum is found at (1, .368) which corresponds to (1, (1/e)). confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:37:35 Again the calculator is useful but it doesn't replace analysis. You have to do the analysis for this problem and document it. Critical points occur when the derivative is 0. Applying the product rule you get x' e^(-x) + x (e^-x)' = 0. This gives you e^-x + x(-e^-x) = 0. Factoring out e^-x: e^(-x) (1-x) = 0 e^(-x) can't equal 0, so (1-x) = 0 and x = 1. Now, for 0 < x < 1 the derivative is positive because e^-x is positive and (1-x) is positive. For 1 < x the derivative is negative because e^-x is negative and (1-x) is negative. So at x = 1 the derivative goes from positive to negative, indicating the the original function goes from increasing to decreasing. Thus the critical point gives you a maximum. The y value is 1 * e^-1. The extremum is therefore a maximum, located at (1, e^-1). **
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RESPONSE --> Instead of relying on the calculator alone, I should have further substantiated my answer by doing the actual math behind it.
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11:37:41 4.3.42 (formerly 4.3.40) (was 4.2 #38) memory model p = (100 - a) e^(-bt) + a, a=20 , b=.5, info retained after 1, 3 weeks.How much memory was maintained after each time interval?
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RESPONSE --> p = (100 - a) e^(-bt) + a At t=1, p = (100 - 20) e^(-.5*1) + 20 p=68.5% At t=3, p = (100 - 20) e^(-.5*3) + 20 p=37.9% confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:37:45 Plugging in a = 20, b = .5 and t = 1 we get p = (100 - 20) e^(-.5 * 1) + 20 = 80 * e^-.5 + 20 = 68.52, approx., meaning about 69% retention after 1 week. A similar calculation with t = 3 gives us 37.85, approx., indicating about 38% retention after 3 weeks. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:37:52 ** At what rate is memory being lost at 3 weeks (no time limit here)?
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RESPONSE --> To determine rate of change of p, we take the derivative of both sides. dp/dt=(100-a) * (-b)e^(-bt) + 0 After 3 weeks...(100 - 20) * (-.5)e^(-.5*3)=-8.9% ------------------------------------------------ Self-critique rating #$&*:ent: 1
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11:38:05 The average rate of change of y with respect to t is ave rate = change in y / change in t. This is taken to the limit, as t -> 0, to get the instantaneous rate dy/dt, which is the derivative of y with respect to t. This is the entire idea of the derivative--it's an instantaneous rate of change. The rate of memory loss is the derivative of the function with respect to t. dp/dt = d/dt [ (100 - a) e^(-bt) + a ] = (100-a) * -b e^-(bt). Evaluate at t = 3 to answer the question. The result is dp/dt = -8.93 approx.. This indicates about a 9% loss per week, at the 3-week point. Of course as we've seen you only have about 38% retention at t = 3, so a loss of almost 9 percentage points is a significant proportion of what you still remember. Note that between t = 1 and t = 3 the change in p is about -21 so the average rate of change is about -21 / 2 = -10.5. The rate is decreasing. This is consistent with the value -8.9 for the instantaneous rate at t = 3. **
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RESPONSE --> ------------------------------------------------ Self-critique rating #$&*:ent: 3
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11:38:15 4.2.48 (formerly 4.2.46) (was 4.2 #42) effect of `mu on normal distribution
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RESPONSE --> Of the possible problems given in section 4.2 (42, 46, or 48), none involved the use of mu or 'mu. In fact, none of the problems in 4.2 use mu. In 4.3, however, several problems use mu, so I'm not really sure what problem I should be working. confidence rating #$&*: 1 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:38:20 The calculator should have showed you how the distribution varies with different values of `mu. The analytical explanation is as follows: The derivative of e^[ -(x-`mu)^2 / (2 `sigma) ] is -(x - `mu) e^[ -(x-`mu)^2 / 2 ] / `sigma. Setting this equal to zero we get -(x - `mu) e^[ -(x-`mu)^2 / 2 ] / `sigma = 0. Dividing both sides by e^[ -(x-`mu)^2 / 2 ] / `sigma we get -(x - `mu) = 0, which we easily solve for x to get x = `mu. The sign of the derivative -(x - `mu) e^[ -(x-`mu)^2 / 2 ] / `sigma is the same as the sign of -(x - `mu) = `mu - x. To the left of x = `mu this quantity is positive, to the right it is negative, so the derivative goes from positive to negative at the critical point. By the first-derivative test the maximum therefore occurs at x = `mu. More detail: We look for the extreme values of the function. e^[ -(x-`mu)^2 / (2 `sigma) ] is a composite of f(z) = e^z with g(x) = -(x-`mu)^2 / (2 `sigma). g'(x) = -(x - `mu) / `sigma. Thus the derivative of e^[ -(x-`mu)^2 / (2 `sigma) ] with respect to x is -(x - `mu) e^[ -(x-`mu)^2 / 2 ] / `sigma. Setting this equal to zero we get x = `mu. The maximum occurs at x = `mu. **
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RESPONSE --> Again, still not certain what problem this references, even after seeing the answer.
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11:38:27 Add comments on any surprises or insights you experienced as a result of this assignment.
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RESPONSE --> This entire assignment was a beast and reminded me of all the reasons I've never been a fan of natural logarithms. confidence rating #$&*: 1 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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11:38:43 Typical Comment so if you feel very rusty you'll know you aren't along: Good grief, lol where to start!!! Just kidding! I guess I really need to be refreshed on how to handle deriving the exponential function with e. 4.2 was the killer for me here with only minimum examples in the section I had to review my old text and notes. It's just been so long.
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RESPONSE --> confidence rating #$&*: " ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^