cos(派/5)*cos(2派/5)=?

cos(派/5)*cos(2派/5)=?
cos(派/5)*cos(2派/5)=?

cos(派/5)*cos(2派/5)=?
cos(π/5)cos(2π/5)
=[sin(π/5)cos(π/5)cos(2π/5)]/sin(π/5) {乘一个sin(π/5),再除一个sin(π/5)}
=[(1/2)sin(2π/5)cos(2π/5)/sin(π/5) {由上式子用二倍角公式得到}
=(1/4)sin(4π/5)/sin(π/5) {由上式子再用二倍角公式得到}
因为sin(4π/5)=sin(π/5)
所以(1/4)sin(4π/5)/sin(π/5)=1/4
所以cos(π/5)cos(2π/5)=1/4
（其中我用了两次二倍角公式：2sinXcosX=sin2X）

cos(π/5)*cos(2π/5)=0.81*0.31=0.25

cuole