sin20°*cos10°-cos160°*sin10°
=sin20°*cos10°+ cos20°*sin10° 注：cos160°= cos(180°- 20°) = -cos20°
=sin(20°+10°) 注：sin(α+β)=sinα*cosβ+cosα*sinβ
=sin30°
=1/2

sin(20) = 0.912945251
sin(25) = -0.13235175
cos(25) = 0.991202812

解：sin20°cos20°/cos50°=sin20°cos20°/sin40°=sin20°cos20°/（2sin20°cos20°）=1/2或sin20°cos20°/cos50°=（1/2sin40°）/sin40°=1/2

sin3α=sin(α+2α)=sinαcos2α+cosαsin2α=sinα(1-2sin2α)+2sinα(1-sin2α)