$ f_X (x;\lambda)= \lambda e^{-\lambda x} $
- to find $ \hat \lambda_{ml} $ maximize $ \lambda e^{-\lambda x} $ by taking the derivative
- $ -e^{-\hat \lambda x} - \hat \lambda x e^{-\hat \lambda x} = 0 $
- $ \hat \lambda x = 1 $
- $ \hat \lambda_{ML} = 1/x $
