In this paper numerical integration techniques are used to derive an approximation to $$Q(a\sqrt{\gamma })Q(b\sqrt{\gamma })$$ . Further this approximation is used to solve the symbol error probability (SEP) integral containing $$Q(a\sqrt{\gamma })Q(b\sqrt{\gamma })$$ over various fading distributions. The accuracy of proposed approximation and solution to SEP integral are verified using computer simulations and in-built functions in MATLAB.