cos72度*cos36度等于多少？

2sin36°cos36°=sin(2*36°)=sin72°=sin(180°-72°)=sin108°=sin(3*36°)=3sin36°-4sin??36°=sin36°(3-4sin??36°)=sin36°[3-4(1-cos??36°)]=sin36°(4cos??36°-1)4cos??36°-2cos36°-1=0,cos36°=[2+√(2??+4*4*1)]/(2*4)=(√5+1)/4cos72°cos36°=cos(2*36°)cos36°=(2cos??36°-1)cos36°={2[(√5+1)/4]??-1}(√5+1)/4=(3-√5)/2

cos72°cos36°
=2sina36°cos72°cos36°/2sina36°
=sin72°cos72°/2sin36°
=2sin72°cos72°/4sin36°
=sin144°/4sin36°
=sin36°/4sin36°
=1/4

0.25