# Math Help - discrete probability density function!

1. ## discrete probability density function!

need help with discrete probability density function!

Find the value of a such that f(x)=a2^-x may be used as a discrete probability density function for X is equal or greater than 0

what i did was i thought since its density function so
it should be 1=a2^-x
really dont know how to do the rest!!
2. Solve $\sum\limits_{n = 0}^\infty {\frac{a}{{2^n }}} = 1$ for a.