Using sin(A+B) = (sin A) cos(B) + cos(A) sin(B) and sin(A-B) = (sin A) cos(B) - cos(A) sin(B)
To prove sin ( θ + α ) sin ( θ − α ) = sin^2θ − sin^2a
Taking the LHS,
sin ( θ + α ) sin ( θ − α )
= [sin(θ)cos(a)+cos(θ)sin(a)] [sin(θ)cos(a)-cos(θ)sin(θ)]
=sin^2θcos^2a-cos^2θsin^2a (Expanding and...