Chapter 6.4, Problem 240E

[T] Suppose that ${\displaystyle \underset{n=0}{\overset{\infty }{\sum }}{a}_n{x}^n}$ converges to a function f(x) such that f(0) = 0, f' (0) = 1, and f" (x) = −f(x). Find a formula for a_nand plot the partial sum S_Nfor N = 10 on [−5, 5].