先化简,再求值：（4）4xy-[(x²+5xy-y²)-(x²+3xy-2y²)],其中x=-4分之1、y=-2分之1

先化简,再求值：（4）4xy-[(x²+5xy-y²)-(x²+3xy-2y²)],其中x=-4分之1、y=-2分之1
先化简,再求值：（4）4xy-[(x²+5xy-y²)-(x²+3xy-2y²)],其中x=-4分之1、y=-2分之1

先化简,再求值：（4）4xy-[(x²+5xy-y²)-(x²+3xy-2y²)],其中x=-4分之1、y=-2分之1
原式=4xy-(x²+5xy-y²-x²-3xy+2y²)
=4xy-x²-5xy+y²+x²+3xy-2y²
=(4xy-5xy+3xy)+(-x²+x²)+(y²-2y²)
=2xy-y²
=2×(-1/4)×(-1/2)-2×(-1/2)²
=1/4-1/2
=-1/4

简化后y（2x-y) 代入数值后是0

4xy-[(x^2+5xy-y^2)-(x^2+3xy-2y^2)]
=4xy-(x^2+5xy-y^2-x^2-3xy+2y^2)
=4xy-2xy-y^2
=2xy-y^2
把x=-1/4和y=-1/2代人2xy-y^2=2*(-1/4)*(-1/2)-(-1/2)^2=0
所以所求代数式的值是0