summation - Solving sum of $(-1)^n (1/2)^n$

How to solve the following sum?

$$\sum_{n=0}^k (-1)^n (1/2)^n$$

Answer

use the geometric serie for $|x|<1$
$$\sum_{n=0}^{k }x^n=1+x+x^2+x^3+....=\frac{1-x^{k+1}}{1-x}$$

then use $x=-1/2$