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[TEX]tan{\frac{A+B}{2}}=cot{\frac{C}{2}} \\ \ \Leftrightarrow \frac{tan{\frac{A}{2}}+tan{\frac{B}{2}}}{1-tan{\frac{A}{2}}tan{\frac{B}{2}}}=cot{\frac{C}{2}}\\ \ \Leftrightarrow \frac{cot{\frac{A}{2}}+cot{\frac{B}{2}}}{cot{\frac{A}{2}}.cot{\frac{B}{2}}-1}=cot{\frac{C}{2}}\\ \ \Leftrightarrow cot{\frac{A}{2}}+cot{\frac{B}{2}}=cot{\frac{A}{2}}cot{\frac{B}{2}}cot{\frac{C}{2}}-cot{\frac{C}{2}}\\ \ \Leftrightarrow cot{\frac{A}{2}} +cot{\frac{B}{2}}+cot{\frac{C}{2}}=cot{\frac{A}{2}}cot{\frac{B}{2}}cot{\frac{C}{2}}(dpcm)[/TEX]