分类：教育/科学-学习帮助
问题：三角函数求值：cosл/5*cos（2л/5） 
求值：cл/5 *cos（2л/5）
回答：cos(π/5)cos(2π/5)
={1/[2son(π/5)]}2sin(π/5)cos(π/5)cos(2π/5)
={1/[4sin(π/5)]}2sin(2π/5)cos(2π/5)
=[sin(4π/5)]/[4sin(π/5)]
=[sin(π-π/5)]/[4sin(π/5)]
=sin(π/5)/[4sin(π/5)]
=1/4.