Let X be teh mean of a random sample from the exponential distribution $\displaystyle Exp(\theta)$

Show that X is an unbiased point estimater of $\displaystyle \theta$

Which PDF of the exponential function do I use?

http://upload.wikimedia.org/math/6/e...a2d2ada3ca.png or http://upload.wikimedia.org/math/d/c...ece9bf0783.png