In reply to Alfred Keter.

It depends what operation you are performing. The mixture density is the sum of the component densities:
f(x) = pi_1 * f_1(x) + pi_2 * f_2(x)
The same is true for the CDF.
However, when you generate a random sample, you sample from f_1 with probability pi_1 and from f_2 with probability pi_2. There is no adding.