2a²-3ab+b²-a²+ab-2b²
=2a²-a² -3ab+ab +b²-2b²
=a²-2ab-b²
再往下可以做因式分解
=a²-2ab+b²-2b²
=(a-b)²-(√2b)²
=(a-b+√2b)(a-b-√2b)