Question

Simplify: a) 2sin4x.sin3x​
b) 2cos7x.cos3x

Answer 1

-2sinx siny = cos(x+y) - cos(x-y)⇒ 2sinx siny =  cos(x-y) - cos(x+y)

a) x = 4x, y = 3x

so = cos(4x-3x) - cos(4x+3x)

= cosx - cos7x

b) x = 7x, y = 3x

so = cos(7x-3x) - cos(7x+3x)

= cos4x - cos10x