Derive the indicated result by appealing to the geometric series

$\displaystyle \sum\limits_{k = 0}^\infty$ $\displaystyle (-1)^kx^k =1/(1+x), $ |x| < 1

$\displaystyle \sum\limits_{k = 0}^\infty$ $\displaystyle (-1)^kx^{2k} =1/(1+x^2), $ |x| < 1

I dun understand what the question is asking, the geometric series is 1/(1-x), how does it turn into 1/(1+x)