Đáp án: $= \underbrace{666.....653333......334}_{99 c/s 6; 99 c/s 3}$
Giải thích các bước giải:
$\underbrace{666....66}_{100 c/s 6} . \underbrace{9999.....999}_{100 c/s 9}$
$= \underbrace{666....66}_{100 c/s 6} . (\underbrace{1000....0000}_{100 c/s 0} - 1)$
$= \underbrace{666....66}_{100 c/s 6} . \underbrace{1000...000}_{100 c/s 0} - \underbrace{666....66}_{100 c/s 6}$
$= \underbrace{666....66000....000}_{100 c/s 6; 100 c/s 0} - \underbrace{666....66}_{100 c/s 6}$
$= \underbrace{666.....653333......334}_{99 c/s 6; 99 c/s 3}$
