TeX source:

\text{Var}(x_t) = \text{Var} \left(\sum_{j=0}^{\infty} \phi^j_1w_{t-j} = \sum_{j=0}^{\infty}\text{Var}(\phi^j_1w_{t-j}) = \sum_{j=0}^{\infty}\phi^{2j}_1\sigma^2_w = \sigma^2_w \sum_{j=0}^{\infty}\phi^{2j}_1 = \frac{\sigma^2_w}{1-\phi^2_1} \right)