若a2+3ab=4,5ab+2b2=2,则2a2-9ab-6b2的值为...字母后面的数字是次数

问题描述:

若a2+3ab=4,5ab+2b2=2,则2a2-9ab-6b2的值为
...字母后面的数字是次数

2a2-9ab-6b2
=2a2+6ab-15ab-6b2
=(2a2+6ab)-(15ab+6b2)
=2(a2+3ab)-3(5ab+2b2)
=2×4-3×2
=8-6
=2