(x+y)²-4(x+y-1)

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(x+y)²-4(x+y-1)
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(x+y)²-4(x+y-1)
(x+y)²-4(x+y-1)

(x+y)²-4(x+y-1)
(x+y)²-4(x+y-1)
=(x+y)²-4(x+y)+4
=(x+y-2)²

x²+y²+2xy-4x-4y+4

(x+y-2)的平方

(x+y)²-4(x+y-1)
=(x+y)²-4(x+y)+4
=(x+y-2)²

(x+y)²-4(x+y-1)
=(x+y)²-4(x+y)+4
=(x+y)²-2*2(x+y)+2² 完全平方公式
=(x+y-2)²