(x+y)^2-(x+y)^3
=(x+y)^2［1-(x+y)］
=(1-x-y)(x+y)^2
原式=(x+y)²[1-(x+y)]
=(x+y)²(1-x-y)
解：
(X+Y)^2-(X+Y)^3
=(X+Y)^2(1-X-Y)
（X+Y）2-(X+Y)3
设A=X+Y
则A2-A3
=A2(1-A)
=(X+Y)2(1-X-Y)
(x+y)²-(x+y)³
=(x+y)²[1-(x+y)]
=(x+y)²(1-x-y)
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