sin（-870度）×cos930度÷cos（-1380度）×sin（-690度）

四分之根号三

sin(-870)=sin(-720-150)=-sin150=-1/2
cos930=cos(150-1080)=cos150=-√3/2
cos(-1380)=cos(60-1440)=cos60=1/2
sin(-690)=(30-720)=1/2
原式=-1/2×(-√3/2)÷1/2×1/2
=√3/4

sin（-870度）×cos930度÷cos（-1380度）×sin（-690度）=sin(-720°-150°)*cos(720°+210°)÷cos(-1440°+60°)*sin(-720°+30°)=sin(-150°)*cos(210°)÷cos(60°)*sin(30°)=(-sin30)*(-cos30)÷cos(60°)*s...