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=8 y=-1/2
=3x²y-2xy²+2xy-3x²y-xy+3xy²
=xy-x²y
=xy(1-x)
=8*(-1/2)*(1-8)
=-4*(-7)
=28