Ta có [TEX]\frac{A+B}{2}=\frac{\pi}{2}-\frac{C}{2} \Rightarrow tan {\frac{A+B}{2}}=cot\frac{C}{2}\\ \Leftrightarrow \frac{tan{\frac{A}{2}}+{\frac{B}{2}}}{1-tan \frac{A}{1} tan \frac{B}{2}}=\frac{1}{tan{\frac{C}{2}}} \\ \Leftrightarrow [tan\frac{A}{2}+tan \frac{B}{2}]tan \frac{C}{2}=1-tan \frac{A}{2} tan \frac{B}{2}\\ \Rightarrow dpcm [/TEX]