645b⁴-(a-b)⁴=(5b)⁴-(a-b)⁴=[(5b)²+(a-b)²][(5b)²-(a-b)²]=(25b²+a²-2ab+b²)[(5b+(a-b)][5b-(a-b)]=(a²-2ab+26b²)(a+4b)(6b-a)