多项式2mx的2次方-x的2次方+5x+8-（7x的2次方-3y+5x）与x无关求m的2次方-【2m的2次方-（5m-4

2mx^2-x^2+5x+8-(7x^2-3y+5x) =2mx^2-x^2+5x+8-7x^2+3y-5x =2mx^2-8x^2+3y =(2m-8)x^2+3y 因为与x无关 所以2m-8=0 m=4 m^2-[2m^2-(5m-4)+4+m] =-m^2+4m-8