Science:Math Exam Resources/Courses/MATH100/December 2011/Question 04 (a)

MATH100 December 2011

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Question 04 (a)
Full-Solution Problems. In questions 2-8, justify your answers and show all your work. If a box is provided, write your final answer there. Simplification of answers is not required unless explicitly stated. Let T2(x) be the second degree Taylor polynomial about a = 8 for $f(x)={\sqrt[{3}]{x}}.$ Find T2(x) and simplify your answer.

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 hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Recall that the general forumla for a second degree Taylor polynomial is: $T_{2}(x)=f(a)+f'(a)(x-a)+{\frac {f''(a)}{2}}(x-a)^{2}$

Checking a solution serves two purposes: helping you if, after having used the hint, 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

The general forumla for a second degree Taylor polynomial is: $T_{2}(x)=f(a)+f'(a)(x-a)+{\frac {f''(a)}{2}}(x-a)^{2}$ To find this for ƒ(x), we need to first find ƒ'(x) and ƒ″(x): $f'(x)=({\sqrt[{3}]{x}})'=(x^{1/3})'={\frac {1}{3}}x^{-2/3}$ $f''(x)=({\frac {1}{3}}x^{-2/3})'=-{\frac {2}{9}}x^{-5/3}$ And now we can evaluate at a = 8 to get $f(8)=8^{1/3}=2$ $f'(8)={\frac {1}{3}}8^{-2/3}={\frac {1}{3}}\cdot {\frac {1}{4}}={\frac {1}{12}}$ $f''(8)=-{\frac {2}{9}}8^{-5/3}=-{\frac {2}{9}}\cdot {\frac {1}{2^{5}}}=-{\frac {2}{9\cdot 32}}=-{\frac {1}{144}}$ We can now plug these in to the equation for the second Taylor polynomial and obtain: $T_{2}(x)=2+{\frac {1}{12}}(x-8)-{\frac {1}{288}}(x-8)^{2}$ (This is simplified enough for the purpose of Taylor polynomials).

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