先化简,再求值. [(2x+y)^2-(x+y)^2-(x+y)(x-4y) 其中: x=2分之1,y-2 解:

问题描述:

先化简,再求值. [(2x+y)^2-(x+y)^2-(x+y)(x-4y) 其中: x=2分之1,y-2 解:

[(2x+y)^2-(x+y)^2-(x+y)(x-4y)=4x^2+4xy+y^2-(x^2+2xy+y^2)-(x^2-3xy-4y^2)=4x^2+4xy+y^2-x^2-2xy-y^2-x^2+3xy+4y^2=2x^2+5xy+4y^2当x=1/2,y=2时原式=1/2+5+16=21又1/2