Giải thích các bước giải:
Ta có:
$888...8$ có tổng chữ số là $8n$
Vì $8n+n=9n\quad\vdots\quad 9$
$\to 888....8+n$ chia hết cho $9$
$\to 888....8-9+n$ chia hết cho $9$
$\to 888....8-9+n$ là hợp số
Đáp án:
M là hợp số
b) M = 888...8 - 9 + n
Ta có : 888...8 ( n chữ số ) = 8n
=> 8n + n = 9n chia hết cho 9
Mà 9 chia hết cho 9
=> 8n - 9 + n chia hết cho 9
Hay 888...8 - 9 + n chia hết cho 9
=> M là hợp số
Chúc bn hok tốt !
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