Đáp án: $\dfrac23$
Giải thích các bước giải:
Ta có:
$\dfrac{\dfrac27+\dfrac2{19}-\dfrac2{21}}{\dfrac37+\dfrac3{19}-\dfrac17}$
$=\dfrac{\dfrac27+\dfrac2{19}-\dfrac2{21}}{\dfrac37+\dfrac3{19}-\dfrac3{21}}$
$=\dfrac{2(\dfrac17+\dfrac1{19}-\dfrac1{21})}{3(\dfrac17+\dfrac1{19}-\dfrac1{21})}$
$=\dfrac23$