证明：
(sinα+sinθ)(sinα-sinθ)
=sin²α-sin²θ
=[(1-cos2α)-(1-cos2θ)]/2
=(cos2θ-cos2α)/2
={cos[(α+θ)-(α-θ)]-cos[(α+θ)+(α-θ)]}/2
={[cos(α+θ)cos(α-θ)+sin(α+θ)sin(α-θ)]-[cos(α+θ)cos(α-θ)-sin(α+θ)sin(α-θ)]}/2
=[2sin(α+θ)sin(α-θ)]/2
=sin(α+θ)sin(α-θ)
证毕