If sin−1x=θ+β and sin−1 y=θ−β,then 1+xy=
cos2 θ+cos2 β
Obvisously x=sin(θ+β) and y=sin(θ−β)∴1+xy=1+sin(θ+β) sin(θ−β)=1+sin2 θ−sin2 β=sin2θ+cos2β.