Cho $f, g:[a,b]\longrightarrow\mathbb{R}$ liên tục trên [a,b]. Chứng minh rằng
$\displaystyle{\min_{a\leq x\leq b}|g(x)|} \left(\int_{a}^{b}|f(x)|\,d x\right)\leq\int_{a}^{b}|f(x)g(x)|\,d x\leq \displaystyle\max_{a\leq x\leq b}|g(x)|\left(\int_{a}^{b}|f(x)|\,d x\right)$
