For the production function $f(x, y)=60 x^{3 / 4} y^{1 / 4}$ considered in Example $8,$ think of $f(x, y)$ as the revenue when $x$ units of labor and $y$ units of capital are used. Under actual operating conditions, say, $x=a$ and $y=b, \frac{\partial f}{\partial x}(a, b)$ is referred to as the wage per unit of labor and $\frac{\partial f}{\partial y}(a, b)$ is referred to as the wage per unit of capital. Show that
$$
f(a, b)=a \cdot\left[\frac{\partial f}{\partial x}(a, b)\right]+b \cdot\left[\frac{\partial f}{\partial y}(a, b)\right]
$$
(This equation shows how the revenue is distributed between labor and capital.)