Which function does ∑n=0∞(−1)nx2n\sum_{n=0}^{\infty} (-1)^n x^{2n}∑n=0∞(−1)nx2n represent?
11−x\frac{1}{1-x}1−x1
11+x\frac{1}{1+x}1+x1
11+x2\frac{1}{1+x^2}1+x21
11−x2\frac{1}{1-x^2}1−x21