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