奇变偶不变。cos(5π/6-a)=cos[π-（π/6+a）]=cos[-(π/6+a)]
cos位于第四象限是正的。所以cos[-(π/6+a)]=cos(π/6+a)
cos(5π/6-a)=cos(π-a-π/6)=-cos(-a-π/6) (偶函数)=-cos(a+π/6)
cos(5π/6-α)=-cos(π-5π/6+α)=-cos(π/6+α)
