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The series 
1-x^(2)+(x^(4))/(2!)-(x^(6))/(3!)+(x^(8))/(4!)+cdots+(-1)^(n)(x^(2n))/(n!)+cdots converges to which of the following?
(A) 
cos(x^(2))+sin(x^(2))
(B) 
1+x sin x
(C) 
cos x
(D) 
e^(-x^(2))

The series 1x2+x42!x63!+x84!++(1)nx2nn!+ 1-x^{2}+\frac{x^{4}}{2!}-\frac{x^{6}}{3!}+\frac{x^{8}}{4!}+\cdots+(-1)^{n} \frac{x^{2 n}}{n!}+\cdots converges to which of the following?\newline(A) cos(x2)+sin(x2) \cos \left(x^{2}\right)+\sin \left(x^{2}\right) \newline(B) 1+xsinx 1+x \sin x \newline(C) cosx \cos x \newline(D) ex2 e^{-x^{2}}

Full solution

Q. The series 1x2+x42!x63!+x84!++(1)nx2nn!+ 1-x^{2}+\frac{x^{4}}{2!}-\frac{x^{6}}{3!}+\frac{x^{8}}{4!}+\cdots+(-1)^{n} \frac{x^{2 n}}{n!}+\cdots converges to which of the following?\newline(A) cos(x2)+sin(x2) \cos \left(x^{2}\right)+\sin \left(x^{2}\right) \newline(B) 1+xsinx 1+x \sin x \newline(C) cosx \cos x \newline(D) ex2 e^{-x^{2}}
  1. Identify general term: Identify the general term of the series.\newlineThe general term of the series is (1)n(x2n)/(n!)(-1)^n(x^{2n})/(n!). This resembles the Taylor series expansion of exe^{x} but with xx replaced by x2-x^2.
  2. Recognize represented function: Recognize the function represented by the series. The series is the Taylor series for ex2e^{-x^2}, since replacing xx in the exe^{x} series with x2-x^2 gives us the terms of the given series.
  3. Match series to function: Match the series to the correct function from the options.\newlineThe series matches the Taylor series for e(x2)e^{(-x^2)}, which is option (D)(D).

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