[Punk]

75. Suppose $u(x), G(x)$ are smooth functions (i.e$.$ they can be differentiated arbitrary number of times). Show that if
\[
u'(x)\leq G(x)u(x)\qquad\text{for all $x>0$},
\]
and $u(0)=0$, then $u(x)\leq0$ for all $x>0$.