cos77°cos17°+sin77°sin17°=cos(77°-17°)=cos60°=1/2
cosαcosβ=2分之根号3,-sinαsinβ
cosαcosβ+sinαsinβ=2分之根号3
cos(α-β)=√3/2
可以取α=π/6,β=0
答案很多,只有满足 α-β=2kπ±π/6,k∈Z 即可