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

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

人气：280 ℃　时间：2020-06-28 08:35:12

解答

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