⑴sin105°
=sin(60°+45°)
=sin60°cos45°+cos60°sin45°
=(√6＋√2）/4
⑵sin165°
=sin15°
=sin(45°-30°)
=sin45°cos30°-sin30°cos45°
=√2/2*√3/2-1/2*√2/2
=(√6-√2)/4
⑶sin13°cos17°+cos13°sin17°
=sin(13°+17°)
=sin30°
=1/2
⑷sin70°cos25°—sin25°cos70°
=sin(70°-25°)
=sin45°
=√2/2
sinx=2/3,cosy=-3/4,且X,Y都是第二象限的角
cosx=-√5/3,siny=√7/4
sin（x+y）
=sinxcosy+cosxsiny
=2/3*-3/4+(-√5/3)*√7/4
=-1/2-√35/12
sin（x-y）
=sinxcosy-cosxsiny
=2/3*-3/4-(-√5/3)*√7/4
=-1/2+√35/12