Bất đẳng thức Trê-bư-sép:Cho tam giác $ABC$. Chứng minh:
a) $\frac{ab}{p-c}+\frac{bc}{p-a}+\frac{ca}{p-b}\geq 4p$
b) $\frac{\sin A+\sin B+\sin C}{\cos A+\cos B+\cos C}\leq \frac{1}{3}(\tan A+\tan B+\tan C)$
c) $\frac{a}{h_{a}+h_{b}}+\frac{b}{h_{c}+h_{a}}+\frac{c}{h_{a}+h_{b}}\leq \frac{\sqrt{3}}{2}.\frac{R}{r}$