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已知a-b=3,那么a3-b3-9ab的值是(  )
A. 3
B. 9
C. 27
D. 81
人气:450 ℃ 时间:2020-06-20 10:43:11
解答
a3-b3-9ab,
=(a-b)(a2+b2+ab)-9ab,
=(a-b)[(a-b)2+3ab]-9ab,
=3(9+3ab)-9ab,
=27+9ab-9ab,
=27.
故选C.
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