# Math Help - Question relating to Markov's inequality

1. ## Question relating to Markov's inequality

I've been given a question as follows: if Y is a non-negative random variable, I want to show that

$P(X\geq a)\leq e^{-ta}M(t)$,

where M(t) is the moment generating function of X. I think that Markov's inequality would be useful here, and I know that I can express E(X) as $M'(0)$, but I'm not sure how to do this without any more information about X and its mgf.

Any help would be appreciated!

2. Originally Posted by anabelle
I've been given a question as follows: if Y is a non-negative random variable, I want to show that

$P(X\geq a)\leq e^{-ta}M(t)$,

where M(t) is the moment generating function of X. I think that Markov's inequality would be useful here
Notice that $X\geq a$ if, and only if $e^X\geq e^a$. Thus, $P(X\geq a)=P(e^X\geq e^a)$ and apply Markov's inequality now. By the way, no non-negativity is needed here.

3. what about the t?

4. Originally Posted by matheagle
what about the t?
Good point. Let me rephrase:

Originally Posted by Laurent
Let $t>0$. Notice that $X\geq a$ if, and only if $e^{tX}\geq e^{ta}$. Thus, $P(X\geq a)=P(e^{tX}\geq e^{ta})$ and apply Markov's inequality now. By the way, no non-negativity is needed here, except for $t$.