sin(-1200)
=sin(-4*360+240)
=sin240
=sin(180+60)
=-sin60
=-√3/2
cos1209+cos（-1020)-sin(-1050）+tan855°
=cos(360*3+129)+cos1020+sin1050+tan(4*180+135)
=cos129+cos(3*360-60)+sin(3*360-30)+tan135
=cos129+cos60-sin30+tan135
=cos129+1/2-1/2-1
=cos129-1