Question

已知cos（π/6+α）=根号3/3，求cos（5π/6-α）的值

Answer 1

cos（π/6+α）=根号3/3
cos(-π/6-a)=√3/3
所以
cos(π-π/6-a)=cos(5π/6-a)=-cos(π/6-a)=-√3/3

Answer 2

cos（π/6+α）=根号3/3 cos（-π/6-α）=根号3/3 - cos（-π/6-α+π）=根号3/3
cos（5π/6-α）=-根号3/3