已知 a²-2ab=3,b²-ab=4,那么2b²-a²的值是多少?快些,thank you.

已知 a²-2ab=3,b²-ab=4,那么2b²-a²的值是多少?快些,thank you.
已知 a²-2ab=3,b²-ab=4,那么2b²-a²的值是多少?
快些,thank you.

已知 a²-2ab=3,b²-ab=4,那么2b²-a²的值是多少?快些,thank you.
2（b²-ab）-（ a²-2ab）
=2b²-a²
=2*4-3
=5

2b^2-a^2=2*(b^2-ab)-(a^2-2ab)
=2*(4)-3
=8-3
=5

由b²-ab=4得
2b²-2ab=8，2ab=2b²-8
∴ a²-2ab=a²-2b²+8=3
∴2b²-a²=8-3=5