# Find the sum up to $n$ terms of the G.P. $1,-a,a^2,-a^3.........$
$\begin{array}{1 1}\large\frac{1-(A)^n}{1-a} \\ \large\frac{1-(-a)^n}{1+a} \\\large\frac{1-(-a)^n}{1-a} \\ \large\frac{1+(-a)^n}{1+a} \\ \large\frac{1+(-a)^n}{1+a} \end{array}$