$$\frac{\partial \log L}{\partial \lambda} = \sum_{i=1}^{n} \frac{x_i}{\lambda} - n = 0$$
$$\hat{\sigma}^2 = \frac{1}{n} \sum_{i=1}^{n} (x_i-\bar{x})^2$$ theory of point estimation solution manual
$$\frac{\partial \log L}{\partial \sigma^2} = -\frac{n}{2\sigma^2} + \sum_{i=1}^{n} \frac{(x_i-\mu)^2}{2\sigma^4} = 0$$ theory of point estimation solution manual
Suppose we have a sample of size $n$ from a normal distribution with mean $\mu$ and variance $\sigma^2$. Find the MLE of $\mu$ and $\sigma^2$. theory of point estimation solution manual
Solving this equation, we get: