cosα+cosθ=-cosγ.式1.
sinα+sinθ=-sinγ .式2.
(式1)^2+(式2)^2=2+2cosαcosθ+2sinαsinθ=1
cos(θ-α)=-1/2.
因0<α<β<γ<π,所以θ-α<π,而cos(θ-α)