已知a-b=3,那么a3-b3-9ab的值是( ) A.3 B.9 C.27 D.81
问题描述:
已知a-b=3,那么a3-b3-9ab的值是( )
A. 3
B. 9
C. 27
D. 81
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.