Question

若n为正奇数，则7ⁿ+7^n-1+7^n-2+…+7被9除的余数是(    )

A.0                B.2                  C.7                D.8

Answer 1

解析：原式=7ⁿ+7^n-1+7^n-2+…+·7+1-1

=

=·9ⁿ-9^n-1+…+·9·(-1)^n-1+（-1）^n-1=9ⁿ-·9^n-1+…+·9-2(n为正奇数)，所以被9除，余数为7.

答案：C