sinθ+cosθ=2sina 1
sinθcosθ=sin^2b
1式平方
sin^2 θ+2sinθcosθ+cos^2=4sin^2 a
1+2sin^2b=4sin^2a
1+1-cos2b=2(1-cos2a)=2-2cos2a
2cos2a=cos2b