已知|x|=5,y=-8,求多项式-1/2(3xy-2x的2次方)+3(x的二次方-1/2xy)的值.-1/2(3xy-2x的2次方)+3(x的二次方-1/2xy)=4x²-3xy=100+24xx=5时,-1/2(3xy-2x的2次方)+3(x的二次方-1/2xy)=220x=-5时,-1/2(3xy-2x的2次方)+...