CMR trong mọi tam giác ABC
a) r + ra + rb - r = 4R.cosC
b)tan\(\frac{B}{2}\). tan \(\frac{C}{2}\) = \(\frac{h_a-2r}{h_a}\) = \(\frac{h_a}{2r_a+h_a}\)
c) cos\(\frac{A}{2}\) = \(\sqrt{\frac{p\left(p-a\right)}{bc}}\) ; tan\(\frac{A}{2}\) = \(\sqrt{\frac{\left(p-b\right)\left(p-c\right)}{p\left(p-a\right)}}\)