Integrais

$\int_0^{\infty} e^{-ax^2} dx = {1\over 2} \sqrt {\pi\over a}$

$\int_0^{\infty} x e^{-ax^2} dx = {1\over 2a}$

$\int_0^{\infty} x^2 e^{-ax^2} dx = {{\sqrt{\pi}}\over {4a^{3/2}}}$

$\int_0^{\infty} x^3 e^{-ax^2} dx = {1\over {2a^2}}$

$\int_0^{\infty} x^4 e^{-ax^2} dx = {3\over {8a^2}} \sqrt {\pi\over a}$