Đáp án:
Giải thích các bước giải:
a) Ta có: $abc\vdots$ $7$
Mà $abc= 100a+ 10b+ c\vdots$ $7$
$= 98a+ 2a+ 7b+ 3b+ c\vdots$ $7$
$= (98a+ 7b)+ (2a+ 3b+ c)\vdots$ $7$
$= 7. (14a+ 1b)+ (2a+ 3b+ c)\vdots$ $7$
Vì $3. (14a+ 1b)\vdots$ $7⇒ (2a+ 3b+ c\vdots$ $7)$
Vậy $2a+ 3b+ c\vdots$ $7$
$b) abcd= 100ab+ cd= 99ab+ ab+ cd$
$99ab\vdots$ $11; ab+ cd\vdots$ $11$
$⇒ abcd\vdots$ $11$
$⇒ đpcm$