cos20cos40cos80
=sin20cos20cos40cos80/sin20
=(1/2)sin40cos40cos80/sin20
=(1/4)sin80cos80/sin20
=(1/8)sin160/sin20
=1/8
sin70*sin50*sin10=-1/2(cos120-cos20)*sin10=1/2(sin10cos20+1/2sin10)=1/2[1/2(sin30-sin10)+1/2sin10]=1/4*sin3=1/8
[[[或者有个三倍角公式直接可以算出来
sin3x=4sinx*sin(60+x)*sin(60-x) ]]]
sin30*cos60*sin90=1/4
所以原式=1/8*1/8*1/4=1/256