若sin（6分之π-角二法）=a,则cos（3分之2π-角二法）=?

题目

若sin（6分之π-角二法）=a,则cos（3分之2π-角二法）=?

答案

sin(π/6－α)＝a
cos(2π/3－α)＝－cos(π－2π/3＋α)＝－cos(π/3＋α)＝－cos(π/2－(π/6－α))＝－sin(π/6－α)＝－a