求值:sin(2π/5)cos(π/10).
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.