=a^2b-a^2c+b^2c-b^2a+c^2a-c^2b

=ab(a-b)+bc(b-c)+ac(c-a)

=b(a^2-ab+cb-c^2)+ac(c-a)

=b[(a+c)(a-c)-b(a-c)]+ac(c-a)

=b(a-c)(a+b-c)-ac(a-c)

=(a-c)(ab+b^2-bc-ac)

=(a-c)(a-b)(b-c)=0

So a-c=0 or a-b=0 or b-c=0.

That is, at least two of the three numbers A, B and C are equal.

Method 2:

To expand the formula, you must

Answer? b-a？ c+b？ c-b？ a+c？ b-c？ a=0

(a-b)(c？ -ab-ac+ab)=0

(a-b)(b-c)(c-a)=0

So: at least two of the three numbers A, B and C are equal.

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