Ta có $\widehat{A}+\widehat{B}+\widehat{C}=180 \Rightarrow \frac{\widehat{A}}{2} + \frac{\widehat{B}}{2} +\frac{\widehat{C}}{2} =90^{\circ}$
a) $\sin \frac{A}{2}=\sin ( 90^{\circ} - \frac{B+C}{2} )=\cos \frac{B+C}{2}$ (đpcm)
b) $\tan \frac{B+C}{2}=\tan (90^{\circ} - \frac{A}{2})=\cot \frac{A}{2}$ (đpcm)