If 1,x,x2,x3,…1, x, x^2, x^3, \dots1,x,x2,x3,… is a geometric sequence, what is the value of ∑k=0∞x2k\sum_{k=0}^{\infty} x^{2k}∑k=0∞x2k?
11−x\frac{1}{1-x}1−x1
11−x2\frac{1}{1-x^2}1−x21
11+x\frac{1}{1+x}1+x1
11+x2\frac{1}{1+x^2}1+x21