A function $\displaystyle f(x,y,z)$ is called homogeneous of degree n if $\displaystyle f(tx,ty,tz) = t^nf(x,y,z)$. For example, $\displaystyle z^2\ln{\left(\frac{x}{y}\right)}$ is homogeneous of degree 2 since

$\displaystyle (tz)^2\ln{\left(\frac{tx}{ty}\right)} = t^2\left(x^2\ln{\frac{x}{y}}\right)$

Euler's theorem on homogeneous functions says that if $\displaystyle f$ is homogeneous of degree n, then

$\displaystyle x\frac{\partial{f}}{\partial{x}} + y\frac{\partial{f}}{\partial{y}} + z\frac{\partial{f}}{\partial{z}} = nf$

Prove this theorem.

Can someone just get me started?