course MTH 174 6:10 p.m. 6/6/10 If your solution to stated problem does not match the given solution, you should self-critique per instructions at
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18:28:51
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Note: Problem numbering is according to the problems as presented in Problem Assignments . The numbering will differ from that in your text. ********************************************* Question: Section 6.1, Problem 5 [[6.1.5 (previously 6.1 #12)]] f '(x) =1 for x on the interval (0,2), -1 on (2,3), 2 on (3,4), -2 on (4,6), 1 on (6,7) f(3) = 0 What was your value for the integral of f '? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: By forming the sum of the areas under f'(x) we obtain the value of the integral over the interval [0,7]. 2*1 + 1*-1 + 2*1 + 1*-2 + 2*-2 + 1*1= 2-1+2-2-4 1+1= 0. Confidence Rating #$&*: 3 Given Solution: the change in f from x=0 to x=2 is 2 (area beneath line segment from x=0 to x=1), then from x=2 to x=3 is -1, then from x=3 to x=4 is +2, then from x=4 to x=6 is -4, then from x=6 to x=7 is +1. If f(3) = 0 then f(4) = 0 + 2 = 2, f(6) = 2 - 4 = -2 and f(7) = f(6) + 1 = -1. Working back from x=3, f(2) = 0 - (-1) = 1 and f(0) = 1 - 2 = -1. The integral is the sum of the changes in f ' which is 2 - 1 + 2 - 4 + 1 = 0. Alternatively since f(0) = -1 and f(7) = -1 the integral is the difference f(7) - f(0) = -1 - (-1) = 0. Let me know if you disagree with or don't understand any of this and I will explain further. Let me know specifically what you do and don't understand. **** Alternative solution: Two principles will solve this problem for you: 1. The definite integral of f' between two points gives you the change in f between those points. 2. The definite integral of f' between two points is represented by the area beneath the graph of f' between the two points, provided area is understood as positive when the graph is above the x axis and negative when the graph is below. We apply these two principles to determine the change in f over each of the given intervals. Answer the following questions: What is the area beneath the graph of f' between x = 0 and x = 2? 2 What is the area beneath the graph of f' between x = 3 and x = 4? 2 What is the area beneath the graph of f' between x = 4 and x = 6? -4 What is the area beneath the graph of f' between x = 6 and x = 7? 1 What is the change in the value of f between x = 3 and x = 4? 2 Since f(3) = 0, what therefore is the value of f at x = 4? 2 Now that you know the value of f at x = 4, what is the change in f between x = 4 and x = 6, and what therefore is the value of f at x = 6? -4 -2 Using similar reasoning, what is the value of f at x = 7? -1 Then using similar reasoning, see if you can determine the value of f at x = 2 and at x = 0.** f(2)= 1, f(0)= -1 STUDENT QUESTION: I did not understand how to obtain the value of f(0), but I found that f(7) was 10 by adding all the integrals together INSTRUCTOR RESPONSE: The total area is indeed 10, so you're very nearly correct; however the integral is like a 'signed' area--areas beneath the x axis make negative contributions to the integral--and you added the 'absolute' areas &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): OK ------------------------------------------------ Self-critique rating #$&* 3 ********************************************* Question: Describe your graph of f(x), indicating where it is increasing and decreasing and where it is concave up, where it is straight and where it is concave down. Your solution: The graph is increasing from 0 to 2, then decreasing from 2 to 3, increasing from 3 to 4, decreasing from 4 to 6, and lately increasing from 6 to 7. Each interval is linear and there is therefore no concavity. Confidence Rating #$&*: 3
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Given Solution: ** The graph of f(x) is increasing, with slope 1, on the interval (0,2), since f'(x) = 1 on that interval, decreasing, with slope -1, on the interval (2,3), where f'(x) = -1, increasing, with slope +2, on the interval (3,4), where f'(x) = +2, decreasing, with slope -2, on the interval (4,6), where f'(x) = -2, and increasing, with slope +1, on the interval (6,7), where f'(x) = +1. The concavity on every interval is zero, since the slope is constant on every interval. Since f(3) = 0, f(4) = 2 (slope 2 from x=3 to x=4), f(6) = -2 (slope -2 from x = 4 to x = 6), f(7) = -1 (slope +1 from x=6 to x=7). Also, since slope is -1 from x=2 to x=3, f(2) = +1; and similar reasoning shows that f(0) = -1. ** ** The definite integral of f'(x) from x=0 to x=7 is therefore f(7) - f(0) = -1 - (-1) = 0. ** ** Basic principles: 1. The slope of the graph of f(x) is f'(x). So the slope of your f graph will be the value taken by your f' graph. 2. Note that if the slope of the f graph is constant for an interval that means that the graph is a straight line on the interval. Using these principles answer the following questions: What is the slope of the f graph between x = 0 and x = 2? 1 What is the slope of the f graph between x = 3 and x = 4? 2 What is the slope of the f graph between x = 4 and x = 6? -2 What is the slope of the f graph between x = 6 and x = 7? 1 Given that f(3) = 0 and using the value of the slope of the f graph between x = 3 and x = 4, describe the f graph between these two points. **The graph is linear ascending with a slope of 2. Using similar information describe the graph for each of the other given intervals. ** For (0,2): linear increasing with slope of 1, (2,3) linear decreasing with slope of -1, (4,6) linear decreasing with slope -2, (6,7) linear increasing with slope 1. Also answer the following: What would have to be true of the f' graph for the f graph to be concave up? Same question for concave down. ** The slope of the f'(x) intervals would have to be linearly increasing or decreasing. ......................................... 18:37:09
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&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): Didn't give slope of each interval in my solution but described the slope in the followup question in the given solution. Self-critique rating #$&* 3 ********************************************* Question: Was the graph of f(x) continuous? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: The graph of f(x) was continuous as there we not breaks in the line connecting each point to the next. Confidence Rating #$&*: 3 Given Solution: ** A function f(x) is continuous at x = a if the limit of the f(x), as x approaches a, exists and is equal to f(a). Is this condition fulfilled at every point of the f(x) graph? ** Yes it is.
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18:37:15
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&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): Didn't use the limit definition of continuity. Self-critique rating #$&* 3 ********************************************* Question: How can the graph of f(x) be continuous when the graph of f ' (x) is not continuous? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: Even though the graph of f'(x) is not continuous it is defined and finite at every point over its domain.
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Given Solution: ** If f ' is constant then the slope of the f(x) graph is constant, so the graph of f(x) must be linear ** &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): OK ------------------------------------------------ Self-critique rating #$&* 3 ********************************************* Question: Section 6.1, Problem 10: The graph of outflow vs. time is concave up Jan 1993 -Sept, peaks in October, then decreases somewhat thru Jan 1994; the inflow starts lower than the outflow, peaks in May, then decreases until January; inflow is equal to outflow around the middle of March and again in late July. **** When was the quantity of water greatest and when least? Describe in terms of the behavior of the two curves. YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: The water quantity was greatest around July because that was when water inflow was much greater than outflow so the water could accumulate best in this period. It was the least around January 1994 when the outflow had been much greater than the inflow which would cause the reservoir to drain significantly. Confidence Rating #$&*: 3
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Given Solution: ** Between any two dates the corresponding outflow is represented by the area under the outflow curve, and the inflow by the area under the inflow curve. When inflow is greater than outflow the quantity of water in the reservoir will be increasing and when the outflow is greater than the inflow quantity of water will be decreasing. We see that the quantity is therefore decreasing from January 93 through sometime in late February, increasing from late February through the beginning of July, then again decreasing through the end of the year.The reservoir will reach a relative maximum at the beginning of July, when the outflow rate overtakes the inflow rate. The amount of water lost between January and late February is represented by the difference between the area under the outflow curve and the area under the inflow curve. This area corresponds to the area between the two graphs. The amount of water gained between late February and early July is similarly represented by the area between the two curves. The latter area is clearly greater than the former, so the quantity of water in the reservoir will be greater in early July than on Jan 1.The loss between July 93 and Jan 94, represented by the area between the two graphs over this period, is greater than the gain between late February and early July, so the minimum quantity will occur in Jan 94. The rate at which the water quantity is changing is the difference between outflow and inflow rates. Specifically the net rate at which water quantity is changing is net rate = inflow rate - outflow rate. This quantity is represented by the difference between the vertical coordinate so the graphs, and is maximized around late April or early May, when the inflow rate most greatly exceeds the outflow rate. The net rate is minimized around early October, when the outflow rate most greatly exceeds the inflow rate. At this point the rate of decrease will be maximized. ** ** When inflow is > outflow the amount of water in the reservoir will be increasing. If outflow is < inflow the amount of water will be decreasing. Over what time interval(s) is the amount of water increasing? ** It is increasing from around March to July. Over time interval(s) is the amount of water decreasing? ** It is decreasing from July to Jan(1994).
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**** When was the quantity of water increasing fastest, and when most slowly? Describe in terms of the behavior of the two curves. ** It was increasing the fastest in April because the area between the inflow and outflow is the largest with inflow being greater and increasing the least in October when outflow was greater than the inflow with the maximum area between the two curves and outflowing being the greater of the two.
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The curve increases most between Jan and Apr and it decreases most between July and October ** What aspect of which graph gives you the rate at which water is flowing into the reservoir? The line graphing the inflow function. What aspect of which graph gives you the rate at which water is flowing out of the reservoir? The line graphing the outflow function. What has to be true of the two graphs in order for the amount of water in the reservoir to the increasing at an increasing rate? The inflow slope must be increasing at an increasing rate and the outflow slope decreasing at an increasing rate. What has to be true of the two graphs in order for the amount of water in the reservoir to the increasing at a decreasing rate? The inflow slope will be increasing at a decreasing rate and the outflow will be decreasing at a decreasing rate. What has to be true of the two graphs in order for the amount of water in the reservoir to be decreasing at an increasing rate? The inflow must be decreasing at an increasing rate and the outflow increasing at an increasing rate. What has to be true of the two graphs in order for the amount of water in the reservoir to be decreasing at a decreasing rate? ** The inflow must be decreasing at a decreasing rate and the outflow must be increasing at a decreasing rate.
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18:47:04
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&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): OK ------------------------------------------------ Self-critique rating #$&* 3 ********************************************* Question: Section 6.2, Problem 5 [[6.2.5 (previously 6.2 #26)]] antiderivative of f(x) = x^2, F(0) = 0 YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: The antiderivative is F(x)= 1/3 x^3 + c with c= 0 for F(0)= 0. confidence rating #$&* 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Given Solution: ** An antiderivative of x^2 is x^3/3. The general antiderivative of x^2 is F(x) = x^3/3 + c, where c can be anything. There are infinitely many possible specific antiderivative. However only one of them satisfied F(0) = 0. We have F(0) = 0 so 0^3/3 + c = 0, or just c = 0. The antiderivative that satisfies the conditions of this problem is therefore F(x) = x^3/3 + 0, or just F(x) = x^3/3. ** &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): OK Self-critique rating #$&* 3 ********************************************* Question: Section 6.2, Problem 8 [[(previously 6.2 #56)]] indef integral of t `sqrt(t) + 1 / (t `sqrt(t)) **** What did you get for the indefinite integral? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: Int( t*sqrt(t) + 1/(t*sqrt(t) dt)= Int(t^3/2+ t^-3/2 dt)= (2/5)*t^(5/2) - 2*t^(-1/2) + c= (2/5)*t^2*sqrt(2) - 2/sqrt(2) + c. Confidence Rating #$&*: OK Given Solution: ** The function can be written t^(3/2) + t^(-3/2). Both are power functions of the form t^n. Antiderivative is 2/5 * t^(5/2) - 2 t^(-1/2) + c or 2/5 t^(5/2) - 2 / `sqrt(t) + c. ** (2/5) t^(5/2) + ln(t^3/2)
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&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): I'm not sure where (2/5) t^(5/2) + ln(t^3/2) comes from.