先化简再求值:2m(m-n)-(m+2n)(2m-n),其中m=1,n=-2

分类: 作业答案 • 2021-12-19 11:03:22

问题描述：

答

2m(m-n)-(m+2n)(2m-n)=(2m^2-2mn)-(2m^2+3mn-2n^2)=-5mn+2n^2=n(2n-5m)=(-2)*(-2*2-5*1)
=(-2)*(-9)=18

答

2m(m-n)-(m+2n)(2m-n)
=2m^2-2mn-(2m^2+3mn-2n^2)
=2m^2-2mn-2m^2-3mn+2n^2
=2m^2-2m^2+2n^2-2mn-3mn
=2n^2-5mn
=2*(-2)^2-5*1*(-2)
=8+10
=18