Science:Math Exam Resources/Courses/MATH103/April 2009/Question 05 (b)

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MATH103 April 2009

  •  Q1 (a)  •  Q1 (b)  •  Q1 (c)  •  Q1 (d)  •  Q1 (e)  •  Q1 (f)  •  Q2  •  Q3 (a)  •  Q3 (b)  •  Q4 (a)  •  Q4 (b)  •  Q4 (c)  •  Q5 (a)  •  Q5 (b)  •  Q5 (c)  •  Q6 (a)  •  Q6 (b)  •  Q7 (a)  •  Q7 (b)  •  Q7 (c)  •  Q8 (a)  •  Q8 (b)  •  Q8 (c)  •  

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Question 05 (b)
A carrot has the shape formed by revolving the region under the graph of $f(x)={\sqrt {14-x}}$ for $0\leq x\leq 12$ cm about the x axis. The concentration of vitamin A is found to vary in the carrot with x according to $c(x)={\frac {1}{12}}e^{-x/12}{\frac {\text{mg}}{{\text{cm}}^{3}}}$ What is the total amount of vitamin A in the carrot?

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
The amount of Vitamin A is concentration x volume.

Hint 2
You need to multiply the concentration and the disk-volume for every infinitly small disk and then integrate over all disks.

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

To find the amount A of Vitamin A in the carrot, we need to multiply every infinitly small disk with unit $\ cm^{3}$ with the passend concentration of Vitamin A and then integrate over all disks: ${\begin{aligned}A&=\int _{0}^{12}{\frac {1}{12}}e^{-{\frac {x}{12}}}\left({\sqrt {14-x}}\right)^{2}\pi dx\\&={\frac {\pi }{12}}\int _{0}^{12}e^{-{\frac {x}{12}}}(14-x)dx\\&={\frac {\pi }{12}}\left[\int _{0}^{12}14e^{-{\frac {x}{12}}}dx-\int _{0}^{12}xe^{-{\frac {x}{12}}}dx\right]\end{aligned}}$ We let the former integral as is for now and use integration by parts for the latter. Let u = x, du = dx, dv = e-x/12dx and v = -12e-x/12. Then ${\begin{aligned}A&={\frac {\pi }{12}}\left[\int _{0}^{12}14e^{-{\frac {x}{12}}}dx-\int _{0}^{12}xe^{-{\frac {x}{12}}}dx\right]\\&={\frac {\pi }{12}}\left[\int _{0}^{12}14e^{-{\frac {x}{12}}}dx-\left(\left.x(-12)e^{-{\frac {x}{12}}}\right|_{0}^{12}-\int _{0}^{12}(-12)e^{-{\frac {x}{12}}}\,dx\right)\right]\\&={\frac {\pi }{12}}\left[14\int _{0}^{12}e^{-{\frac {x}{12}}}\,dx-12\int _{0}^{12}e^{-{\frac {x}{12}}}\,dx+(12)(12)e^{-1}-0\right]\\&={\frac {\pi }{12}}\left[2\int _{0}^{12}e^{-{\frac {x}{12}}}\,dx+(12)(12)e^{-1}\right]\\&={\frac {\pi }{12}}\left[\left.2(-12)e^{-{\frac {x}{12}}}\right|_{0}^{12}+(12)(12)e^{-1}\right]\\&={\frac {\pi }{12}}\left[-2(12)(e^{-1}-1)+(12)(12)e^{-1}\right]\\&=\pi \left[-2e^{-1}+2+12e^{-1}\right]\\&=\pi \left[{\frac {10}{e}}+2\right]\\&={\frac {2\pi (5+e)}{e}}.\end{aligned}}$

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