sin(-1200°)=-sin(3*360° +120° )=-sin120°=-1/2
cos1290°=cos(4*360°-150°)=cos150°=-√3/2
cos(-120)°=cos120°=-1/2
cos(-1020)°=cos1020°=cos(3*360°-60°）=1/2
sin(-1050°)=sin(3*360°-30°）=-sin30°=-1/2
tan855°=tan135°=-1
sin(-1200°)-cos1290°+cos(-120)°+cos(-1020)°-sin(-1050°)+tan855°
=-1/2-（-√3/2）-1/2+1/2-（-1/2）+（-1）=（√3-2）/2