Which function does the power series ∑n=0∞(−1)nx2n\sum_{n=0}^{\infty} (-1)^n x^{2n}∑n=0∞(−1)nx2n represent?
11+x2\frac{1}{1+x^2}1+x21
cos(x)\cos(x)cos(x)
11−x2\frac{1}{1-x^2}1−x21
e−x2e^{-x^2}e−x2