先化简再求值：(3x-y)^2-(2x+y)^2-5x(x-y),其中x=20,y=1

分类: 作业答案 • 2021-12-20 06:05:12

问题描述：

答

(3x-y)^2-(2x+y)^2-5x(x-y)
=(3x-y+2x+y)(3x-y-2x-y)-5x(x-y)
=5x*(x-2y)-5x(x-y)
=5x^2-10xy-5x^2+5xy
=-5xy
=-5*20*1
=-100