Possible Duplicate:
Proof that $x \Phi(x) + \Phi'(x) \geq 0$ $\forall x$, where $\Phi$ is the normal CDF
Let Z be a standard normal random varible. How to prove that:
$P(Z>t)>\frac{1}{\sqrt{2\pi}}\frac{t}{t^{2}+1}e^{-\frac{t^{2}}{2}} $
Thanks for helping me.