CMR tam giác ABC đều nếu: a) $\sin \frac{A}{2}.\sin \frac{B}{2}.\sin \frac{C}{2}$=$\cos A.\cos B.\cos C$
b) $\tan A.\tan B.\tan C= \cot \frac{A}{2}.\cot \frac{B}{2}.\cot \frac{C}{2}$
c) $\sqrt{3}h_{a} + \frac{a}{2} = b+c$
d) $\begin{cases}\sin B +\sin C= 2 \sin A \\ \tan B + \tan C = 2\tan A\end{cases}$