求值 cosπ/9cos2π/9cos4π/9
问题描述：
求值 cosπ/9cos2π/9cos4π/9
还有一题：
2·（根号下1+sin4）-（根号下2-2cos4）
答
1.原式=sin(π/9)*[cos(π/9)cos(2π/9)cos(4π/9)]/sin(π/9)=1/2*[sin(2π/9)cos(2π/9)cos(4π/9)]/sin(π/9)=1/8cos(8π/9)/sin(π/9)=1/82.原式=2*根号下(1+2*sin2cos2)-根号下[2-2*(1-2(sin2)^2)]=2*根号下(si...