作业帮先化简再计算（x+y)^2-4(x+y)(x-y)+3(x-y)^2并求出x=5,y= -4/5时的值.
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先化简再计算（x+y)^2-4(x+y)(x-y)+3(x-y)^2并求出x=5,y= -4/5时的值.
先化简再计算
（x+y)^2-4(x+y)(x-y)+3(x-y)^2
并求出x=5,y= -4/5时的值.
先化简再计算（x+y)^2-4(x+y)(x-y)+3(x-y)^2并求出x=5,y= -4/5时的值.
=((x+y)-(x-y))*((x+y)-3(x-y))
=2y*(4y-2x)
=4y*(2y-x)
带入得到=4*-0.8*(-1.6-5)=4*0.8*6.6=21.12
原式=(x+y-3x+3y)(x+y+x-y)
=(4y-2x)(2x)
=4(2y-x)x
=4*(-8/5-5)*5
=4*(-8-25)
=-33*4
=-132
设x+y=a，x-y=b
(x+y)²-4(x+y)(x-y)+3(x-y)²
=a²-4ab+3b
因式分解，得
a²-4ab+3b
=(a-3b)(a-b)
=[(x+y)-3(x-y)][(x+y)-(x-y)]
=(4y-2x)2y
=8y²-4xy
代入x=5，y=-4/5
原式=8×(-4/5)²-4×5×(-4/5)
=8×16/25+16
=528/25
（x+y)^2-4(x+y)(x-y)+3(x-y)^2
=（x+y)^2-4(x^2-y^2)+3(x-y)^2
=x^2+y^2+2xy-4x^2+4y^2+3x^2+3y^2-6xy
=8y^2-4xy
当x=5，y= -4/5时
原式=528/25
原式=[(x+y)-3(x-y)][(x+y)-(x-y)]
=(4y-2x)2y
=8y²-4xy
=8×(-4/5)²-4×5×(-4/5)
=8×16/25+16
=528/25