3x的平方y-[2xy的平方-2（xy-3/2x的平方y）+xy}+3xy的平方 其中x=3 y=-1/2要步骤
=3x^2y-2xy^2+2(xy-3/2x^2y)-xy+3xy^2
=3x^2y+xy^2+2xy-3x^2y-xy
=xy+xy^2
=3*(-1/2)+3*1/4
=-3/4