Question

如果3a的平方+2ab=8,ab+2b的平方=9求3a的平方-ab-6b的平方的值

Answer 1

由 3a^2 +2ab =8,ab+2b^2 =9,可得

3a^2 -ab -6b^2

= 2a^2 +2ab -3ab -6b^2

= (2a^2+2ab) - 3(ab+2b^2)

= 8 -3×9

= 8 -27

= -19

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