9.9. Let $ f \in L^{1}([0,1]) $ and let $ F(x)=\int_{0}^{x}f(t)dt $. If $ E $ is a measurable subset of $ [0,1] $, show that
(a) $ F(E)=\{y: \exist x \in E , y=F(x)\} $ is measurable.
(b) $ m(F(E)) \leq \int_{E}|f(t)| dt $.
