Here sin(α−β)=a‌sin(θ−β)−b‌sin(θ−γ) sin2(α−β)=a2sin2(θ−β)+b2sin2(θ−α)−2ab‌sin(θ−β)‌sin(θ−α) cos(α−β)=cos(θ−β)‌cos(θ−α)+sin(θ−β)‌sin(θ−α) =ab+sin(θ−β)‌sin(θ−α) 2ab‌cos(α−β)=2a2b2+2ab‌sin(θ−β)‌sin(θ−α) sin2(α−β)+2ab‌cos(α−β)=a2(1−b2)+b2(1−a2)+2a2b2 =a2+b2 Here (α−β)=(θ−β)−(θ−α)