36.1. Let $f(x)=1+x+x^{2}+x^{3}+\ldots$ and $g(x)=1-x+x^{2}-$ $-x^{3}+\ldots$ Compute the products: a) $f(x) f(x)$; b) $f(x) g(x)$.
36.1. Answer: a) $1+2 x+3 x^{2}+4 x^{3}+\ldots$; b) $1+x^{2}+x^{4}+x^{6}+\ldots$