Question

若sin20°cos50°=a,则sin50°sin70°等于

Answer 1

sin50°sin70°=cos20°sin50°
cos20°sin50°-sin20°cos50°=sin（50°-20°）=sin30°=1/2
所以
sin50°sin70°=a+1/2

Answer 2

sin20°cos50°=sin20°sin40°=2sin²20°cos20°
sin50°sin70°=cos40°cos20°=(1-2sin²20°)cos20°=1-2sin²20°cos20°=1-a