Science:Math Exam Resources/Courses/MATH103/April 2011/Question 07 (c)

MATH103 April 2011

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Question 07 (c)
Find the Taylor series for the function $f(x)=x\cos x$. Write your answer in summation notation. (Hint: the Taylor series for $\sin x$ may be helpful.) (Taylor series for $\sin x=x-{\frac {x^{3}}{3!}}+{\frac {x^{5}}{5!}}-{\frac {x^{7}}{7!}}+\dots =\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k+1}}{(2k+1)!}}.$)

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 Taylor series for $\sin(x)$ is $\sin(x)=\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k+1}}{(2k+1)!}}=x-{\frac {x^{3}}{3!}}+{\frac {x^{5}}{5!}}-\ldots$.

Hint 2
The derivative of $\sin(x)$ is $\cos(x)$.

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 1

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Using the fact that $\cos(x)$ is the derivative of $\sin(x)$, we can use the Taylor series of $\sin(x)$ to obtain the Taylor series of $\cos(x)$. Specifically, $\cos(x)={\frac {d}{dx}}\sin(x)=1-{\frac {x^{2}}{2!}}+{\frac {x^{4}}{4!}}-\ldots =\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k}}{(2k)!}}$. Therefore, to find $x\cos(x)$ we multiply the taylor series for $\cos(x)$ by the factor $x$. That is $x\cos(x)=x\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k}}{(2k)!}}=\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k+1}}{(2k)!}}$.

Solution 2

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. The first solution assumes that the Taylor series for $\cos(x)$ is not known in advance. The Taylor series for $\sin(x)$ is known since it was given in the statement of a previous question. If the Taylor series for $\cos(x)$ is known, then we can start the solution from the starting point $\cos(x)=\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k}}{(2k)!}}$. At this point, we multiply the sum by $x$ to obtain $x\cos(x)=x\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k}}{(2k)!}}=\sum _{k=0}^{\infty }(-1)^{k}{\frac {x^{2k+1}}{(2k)!}}$.

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