Question

Prove 2SinAsinB =cos(A-B)-cos(A+B)

Answer 1

Right side of the equation:
cos ( A - B ) - cos ( A + B ) =
= cos A cos B + sin A sin B - ( cos A cos B - sin A sin B ) =
= cos A cos B + sin A sin B - cos A cos B + sin A sin B =
= 2 sin A sin B    -  left side of the equation      ( correct )