cosα*(2cosβ-cosα)+sinα*(2sinβ-sinα)=0
2cosαcosβ-cos^2α+2sinαsinβ-sin^2α=0
2(cosαcosβ+2sinαsinβ)-1=0
cos(α-β)=1/2
cos(β-α)=1/2
β-α=2kπ±π/3(k∈Z)
cosα*(2cosβ-cosα)+sinα*(2sinβ-sinα)=0
2cosαcosβ-cos^2α+2sinαsinβ-sin^2α=0
2(cosαcosβ+2sinαsinβ)-1=0
cos(α-β)=1/2
cos(β-α)=1/2
β-α=2kπ±π/3(k∈Z)