Question

Verify the identity. Show your work.
cos(α - β) - cos(α + β) = 2 sin α sin β

Answer 1

cos (a-b) = cos(a)cos(b) + sin(a)sin(b)
cos (a+b) = cos(a)cos(b) - sin(a)sin(b)
so, cos(a-b) - cos(a+b) =

=cos(a)cos(b) + sin(a)sin(b) - [cos(a)cos(b) - sin(a)sin(b)]
=2sin(a)sin(b)