sin(-2A)=-2sinAcos(-A)

First, start by proving

sin (x + y) = sin(x) cos(y) + cos(x) sin (y)

sin(x+y) and cos(x+y) (http://www.jimloy.com/geometry/trigz.htm)

Then simplify by letting x = y to the traditional double-angle formula:

Finally, substitute -A for x and you'll have your proof.

Hi how you solved?:confused:

Thanks