Find the sum to n terms in the geometric progression 1, -a, a2, -a3 …. (if a ≠ -1)

The given G.P. is \[1,\text{ }-\text{ }a,\text{ }a2,\text{ }-\text{ }a3\text{ }\ldots \text{ }.\]

Here, the initial term \[=\text{ }a1\text{ }=\text{ }1\]

Furthermore, the normal proportion \[=\text{ }r\text{ }=\text{ }\text{ }a\]

We realize that,