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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.

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    In the future, it's better not to edit a question to change it to a different question. It causes confusion with comments, votes, etc that are no longer relevant. You should ask a new question instead.2011-12-08

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