已知coa(π/6-a)=m(m≦1),求cos(5π/6+a) ,sin(2π/3-a)的值
问题描述：
已知coa(π/6-a)=m(m≦1),求cos(5π/6+a) ,sin(2π/3-a)的值
答
cos(π/6-a)=m,
故cos(5π/6+a)
=cos[π-(π/6-a)]
= -cos(π/6-a)
= -m
而
sin(2π/3-a)
=sin[π/2+(π/6-a)]
=cos(π/6-a)
=m