Find the function that equals to $1-x^3+x^6-x^9+ \cdots$ 
Find the function that equals  to $1-x^3+x^6-x^9+ \cdots$ for all $|x| < 1$

I know that $\frac{1}{1+x} = 1-x+x^2-x^3+...$ But I couldn't find the pattern here

Substitute $y=x^{3}$
$$S = 1-y+y^{2}-y^{3}+\cdots$$
$$S = \frac{1}{1+y} = \frac{1}{1+x^{3}}$$
