Đáp án: $S=0$
Giải thích các bước giải:
Ta có :
$S=\left(\dfrac13+\dfrac34-\dfrac{13}{12}\right):\left(\dfrac{12}{23}+\dfrac{123}{234}+\dfrac{1234}{2345}\right)$
$\to S=\left(\dfrac{4}{12}+\dfrac{9}{12}-\dfrac{13}{12}\right):\left(\dfrac{12}{23}+\dfrac{123}{234}+\dfrac{1234}{2345}\right)$
$\to S=\dfrac{4+9-13}{12}:\left(\dfrac{12}{23}+\dfrac{123}{234}+\dfrac{1234}{2345}\right)$
$\to S=0:\left(\dfrac{12}{23}+\dfrac{123}{234}+\dfrac{1234}{2345}\right)$
$\to S=0$
