#$&* course Mth 163 2/28/13 around 8 a.m. 013.
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Given Solution: 2^5 stands for 2 raised to the fifth power; i.e., 2^5 = 2*2*2*2*2. The result of this calculation is 2^5 = 32. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q002. What does 2^3 * 2^5 mean? Is the result of power of 2? If so, what power of 2 is it? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: It means 2 to the 3rd power or 2*2*2 = 8 Multiplied by 2^5 which we just saw 2*2*2*2*2 = 32 So we have 8*32 = 256 confidence rating #$&*: 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: 2^3 * 2^5 means (2*2*2) * (2*2*2*2*2). This is the same as 2*2*2*2*2*2*2*2, or 2^8. When we multiply this number out, we obtain 256. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q003. Why do we say that a^b * a^c = a^(b+c)? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: Because a is raised to the b and c power. So if you add them together then raise it you would get the same answer. confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: We saw in the preceding example that 2^3 * 2^5 stood for a product of three 2's, multiply by a product of five 2's. We saw also that the result was identical to a product of eight 2's. This was one instance of the general rule that when we multiply to different powers of the same number, the result is that number raised to the sum of the two powers. One general way to state this rule is to let a stand for the number that is being raised to the different powers, and let b and c stand for those powers. Then we get the statement a^b * a^c = a^(b+c). &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q004. What does (2^3)^5 mean? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: It means you raise 2 to the 3rd power or 2*2*2 = 8 then you raise 8 to the 5th power or 8*8*8*8*8 = 32768 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: Since 2^3 stands for 2*2*2, it follows that (2^3)^5 means (2^3)*(2^3)*(2^3)*(2^3)*(2^3) = (2*2*2)*(2*2*2)*(2*2*2)*(2*2*2)*(2*2*2) = 2*2*2*2*2*2*2*2*2*2*2*2*2*2*2 = 2^15. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): I plugged both answers in and got the same thing. Just different ways to carry it out ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q005. Why do we say that (a^b)^c = a^(b*c)? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: Because b * c are separated by a parentheses we know that they are multiplied by each other. Therefore, we can say that b * c will give the same answer as raising a to the b power then that answer to the c power. confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: We saw in the last example how (2^3)^5 stands for the product of 5 quantities, each equal to the product of three 2's. We saw how this is equivalent to the product of fifteen 2's, and we saw how the fifteen was obtained by multiplying the exponents 3 and 5. In the present question a^b stands for the quantity a multiplied by itself b times. (a^b)^c stands for the quantity a^b multiplied by itself c times, which is equivalent to multiplying a by itself b * c times. Thus we say that (a^b)^c = a^(b * c). &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q006. According to the law a^b * a^c = a*(b+c), if we multiply 2^5 by 2^-2 what power of 2 should we get? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: According to the law you would get 2^5 * 2^-2 = 2^(5 + -2) = 2^(5-2) = 2^3 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: According to the law, 2^5 * 2^-2 = 2^(5 + -2) = 2^(5-2) = 2^3. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q007. Since as we saw in the preceding question 2^5 * 2^-2 = 2^3, what therefore must be the value of 2^-2? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: The value of 2^-2 must be 2^3 / 2^5. We can simplify by saying 2^-2 = 8/ 32. We get 2^-2 = .25 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: One way of seeing this is to note that 2^5 = 32 and 2^3 = 8, so we have 32 * 2^-2 = 8. Dividing both sides by 32 we get 2^-2 = 8 / 32 = 1/4. We can learn something important if we keep the calculation in powers of 2. If 2^5 * 2^-2 = 2^3, then dividing both sides of the equation by 2^5 we obtain 2^-2 = 2^3/2^5, which is equal to 1/2^2. This shows us why 2^-2 = 1/2^2. QUESTIONABLE STUDENT RESPONSE: .25 INSTRUCTOR COMMENT .25 is of course the value of 2^-2. However I'm not sure you've connected this with the fact that 2^5 * 2^-2 = 2^3, as was the intent of the question. A key word in the given question is 'therefore', which asks you to connect your answer to the fact that 2^5 * 2^-2 = 2^3. STUDENT QUESTION It seems to me to be a lot more work to calculate 2^-2 in the process of solving 32 * 2^-2 = 8 but I understand why it can also be done this way. I am just not sure I understand the need to see it this way. I guess I will understand “why” we need to know this in future problems. INSTRUCTOR RESPONSE The distinction is between using a rule and understanding the reason for the rule. The rule is that a^(-b) = 1 / a^b, so of course it's valid to say that 2^-2 = 1 / 2^2 = 1/4. However just applying the rule doesn't give any insight into why the rule must be as it is. The reason this has to be the rule is that if it isn't, then the calculation of this problem and others like it make no sense. If the other laws of exponents are to be consistent, a^(-b) must by 1 / a^b, as demonstrated by this example. STUDENT QUESTION I just don’t understand this part of our assignments. I went back to the CD to see if I could understand it further, but I can’t seem to find the section. Is there a chapter in our book that would help me to better understand this? INSTRUCTOR RESPONSE We're trying to show why 2^(-n) = 1 / 2^n. Think of it this way: Suppose you have the expression, say, 2^3 / 2^7. We can look at this expression in two ways: 2^3 / 2^7, which by the laws of exponents must be 2^(3 - 7) = 2^(-4). Let's suppose we don't know what 2^(-4) means. We can find out by looking at our original expression in another way: 2^3 / 2^7 = (2 * 2 * 2) / (2 * 2 * 2 * 2 * 2 * 2 * 2) = 1 / (2 * 2 * 2 * 2) = 1 / 2^4. So now we have two ways of writing 2^3 / 2^7. One way is 2^(-4), the other way is 1 / 2^4. We conclude that 2^(-4) = 1 / 2^4. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q008. Why must we say that 2^-n = 1 / 2^n, where n stands for any counting number? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: Because a number raised to a negative power is always the reciprocal of that number. We can plug in a random number to test this out. Let’s say n = 2 We would have 2^-2 = 1/ 2^2 Solving this we would get 2 ^-2 = .25 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: This is because for any number m, we have 2^m * 2^-n = 2^( m + -n) = 2^(m-n), and we also have 2^m * (1 / 2^n) = 2^m / 2^n = 2^(m-n). So whether we multiply 2^m by 2^-n or by 1 / 2^n we get the same result. This shows that 2^-n and 1 / 2^n are the same. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q009. According to the law a^b * a^c = a^(b+c), if we multiply 2^3 by 2^-3 what power of 2 should we get? Since 2^-3 = 1 / 2^3, what number must we get when we multiply 2^3 by 2^-3? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: According to the first law we would get 2^3 * 2^-3 = 2^(3 + -3) = 2^(3-3) = 2^0\ To carry out the second part we would get the equation 2^3 * 2^-3. We first must take 2 to the 3rd power which is 8. Then we must take 2 ^-3 = .125. Then we multiply 8* .125 = 1. confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: 2^3 * 2^-3 = 2^(3 + -3) = 2^(3-3) = 2^0. Since 2^-3 = 1 / 2^3 it follows that 2^3 * 2^-3 = 2^3 * ( 1 / 2^3) = 1. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q010. Continuing the last question, what therefore should be the value of 2^0? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: The value of 2^0 = 1 and should be since the second part of that equation had the same numbers and answer of 1. confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: Since 2^3 * 2^-3 = 2^0 and also 2^3 * 2^-3 = 1 we see that 2^0 must be 1. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q011. How do we solve the equation x^3 = 12? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: To solve for x^3 = 12 or 12 ^(1/3) power. We must take the cubed root of both sides getting 2.289 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: We solve the equation by taking the 1/3 power of both sides: (x^3)^(1/3) = 12^(1/3), then by the law (a^b)^c = a^(bc) we have x^(3 * 1/3) = 12^(1/3), so that x^1 = 12^(1/3) or just x = 12^(1/3), which we can easily enough evaluate with a calculator. We obtain a result of approximately x = 2.29 . STUDENT QUESTION I see in the solution you raise each side to a power of 1/3. Is it also okay to just take the cube root of each side, since that is equivalent? INSTRUCTOR RESPONSE: It's easier to denote fractional exponents using the keyboard than to denote radicals, which is the reason fractional exponents are used in these solutions. You should be able to do it either way, if requested. However either way is fine, and your solution is good. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q012. How do we solve the equation x^(5/2) = 44? YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: To solve this we would take the 5/2 root of 44. Or the 2.5 root of 44 which is 4.543 confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Given Solution: If we take the 2/5 power of both sides we get (x^(5/2))^(2/5) = 44^(2/5) or x^(5/2 * 2/5) = 44^(2/5) or x^1 = 44^(2/5) so that x = 44^(2/5). Evaluating by calculator you should obtain approximately x = 4.54. STUDENT COMMENT Okay, I see now why in question 11, you multiplied each side of the equation by 1/3 to get rid of the power of 3 attached to x. In this problem it would be more difficult to take a 5/2 root of a number. It is much easier to just flip that fraction and raise both sides to that power. (5/2) * (2/5) equals 1, so you wind up with x^1 on one side, which is just x and multiply the other side by 2/5 to get the result of x. INSTRUCTOR RESPONSE You could equally well write this as the 5th root of x^2, or even as the square of the 5th root of x, but the fractional notation is more compact, less cumbersome, in a text-based medium. The fractional notation also makes more sense of the calculation, for the reasons you have noted. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique rating: 3 ********************************************* Question: `q013. In terms of the meaning of 5^7, the meaning of 5^4 and the meaning of 5^11, explain why 5^7 * 5^4 = 5^11. We both know that the rule for multiplying these numbers tells us that 5^7 *5^4 = 5^(7 + 4). You can't explain by quoting this rule, or any rule; you need to explain in terms of the given meanings. YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: I am not sure how to justify this?? I could multiply them together and get a number, but it wouldn’t show 5^11. The only way I know is to show it the way you already have???
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