COS \frac{B+C}{2}\sin \frac{B-C}{2}\cot \frac{A}{2}\] \[=2k\sin \frac{A}{2}\,.\,\,\sin \frac{B-C}{2}\TEXT{ }\text{. }\frac{\cos \frac{A}{2}}{\sin \frac{A}{2}}\] \[=2k\sin \left( \frac{B-C}{2} \RIGHT)\sin \left( \frac{B+C}{2} \right)=2k\left( {{\sin }^{2}}\frac{B}{2}-{{\sin }^{2}}\frac{C}{2} \right)\] or we GET L.H.S. =\[\Sigma 2k\left( {{\sin }^{2}}\frac{B}{2}-{{\sin }^{2}}\frac{C}{2} \right)=0\].