1/4（B-A）
=¼[(a²-2ab+b²)-(a²+2ab+b²)]
=¼(a²-2ab+b²-a²-2ab-b²)
=¼×(-4ab)
=-ab
如果2A-3B+C=0,
那么C=3B-2A
=3(a²-2ab+b²)-2(a²+2ab+b²)
=3a²-6ab+3b²-2a²-4ab-2b²
=a²-10ab+b²