a(a-b)³+2a²(a-b)²-2ab(b-a)²
=a(a-b)³+2a²(a-b)²-2ab(a-b)²
=(a-b)²[a(a-b)+2a²-2ab]
=(a-b)²(3a²-3ab)
=3a(a-b)²(a-b)
=3a(a-b)³
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