4sin20°sin40°sin60°sin80°
4sin(20°)sin(40°)sin(60°)sin(80°)=2sin(20°)sin(40°)√3*sin(80°=(cos(-20°)-cos(60°))√3*sin(80°)=(cos(20°)-1/2)√3*sin(80°)=√3/2*(2sin(100°)+√3)/(2)-(√3*sin(80°))/2=2√3sin(100°)+3-2√3sin(80°)=0,75