Let $(X,\Sigma,\mu)$ be a measure space and $f:X \rightarrow \Re$ a positive measurable function. For all $t > 0$ we define $M_{f}(t)= \{ x \varepsilon X: f(x) > t \} $ and $\varphi_{f}(t) = \varphi(M_{f}(t))$. \newline
Show that
$\int_{X}fd\mu = \int_{0}^{\infty} \varphi_{f}(t)dt$