分类：教育/科学-理工学科-数学
问题：求值:sin(2π/5)cos(π/10). 
回答：解:
[sin(π/5)cos(π/10)]^2
=1/4*[1+cos(π/5)][1-cos(2π/5)]
=1/4*[1+cos(π/5)-cos(2π/5)-cos(π/5)cos(2π/5)]
=1/4*[1+cos(π/5)cos(2π/5)]
=5/16
∴sin(π/5)cos(π/10)=(根5)/4.