tan225°*cos240°*sin(-60°)*tan(-30°)
=tan(180°+45°)*cos(180°+60°)*(-sin60°)*(-tan30°)
=tan45°*(-cos60°)*sin60°*tan30°
=1*(-1/2)*(根号3)/2 *(根号3)/3
=-1/4为什么第二步的正负符号突然会变了(-sin60°)*(-tan30°) 负负得正了！那-cos60呢因为cos(180°+60°)=-cos60°