Question

If sin(A+B)=a and sin(A+C)=b ;then proof that cos2(B-C)-4ab cos(B-C)=1-2a²-2b².​

Answer 1

If sin(A+B)=a and sin(A+C)=b ;then proof that cos2(B-C)-4ab cos(B-C)=1-2a²-2b².