先化简再求值,(2x-3y)^2-(x-2y)(x-5y)-(2x+y)(2x-y)其中x=-2,y=-2/5,求过程!

问题描述:

先化简再求值,(2x-3y)^2-(x-2y)(x-5y)-(2x+y)(2x-y)
其中x=-2,y=-2/5,
求过程!

原式=(4x²-12xy+9y²)-(x²-7xy+10y²)-(4x²-y²)
=4x²-12xy+9y²-x²+7xy-10y²-4x²+y²
=-x²-5xy
=-4-4
=-8

(2x-3y)^2-(x-2y)(x-5y)-(2x+y)(2x-y)=4x^2+9y^2-12xy-(x^2-2yx-5yx+10y^2)-(4x^2-y^2)=4x^2+9y^2-12xy-x^2+2yx+5yx-10y^2-4x^2+y^2=-5xy-x^2当x=-2 y=-2/5时原式=-4-4=-8