Let $f\left(x_{1}, x_{2}\right)$ be a production function with two factors and let $w_{1}$ and $w_{2}$ be their respective prices. Show that the elasticity of the factor share $\left(w_{2} x_{2} / w_{1} x_{1}\right)$ with respect to $\left(x_{1} / x_{2}\right)$ is given by $1 / \sigma-1$.