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=（X^3+3X^2Y+3XY^2+Y^3-X^3-Y^3)/(3(X+Y))
=(3X^2Y+3XY^2)/(3(X+Y))
=(3(X+Y))XY/(3(X+Y))
=XY
（2A+B-C)^2= 2A（2A+B-C）+B（2A+B-C）-C（2A+B-C）
=4A^2+B^2+C^2+4AB-4AC-2BC