$a)$ $(x - \dfrac{5}{3})$ $x$ $\dfrac{9}{13}$ `=` $\dfrac{2}{3}$
$\Leftrightarrow$ $x - \dfrac{5}{3}$ $=$ $\dfrac{2}{3}: $ $\dfrac{9}{13}$
$\Leftrightarrow$ $x - \dfrac{5}{3}$ $=$ $\dfrac{26}{27}$
$\Leftrightarrow$ $x = $ $\dfrac{26}{27} + $ $\dfrac{5}{3}$
$\Leftrightarrow$ $x = $ $\dfrac{71}{27}$
Vậy $x = $ $\dfrac{71}{27}$
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$b$ $X$ x $\dfrac{1}{2} + $ $X$ x $\dfrac{3}{4} = $ $\dfrac{6}{7}$
$\Leftrightarrow$ `X(1/2 + 3/4) = 6/7`
$\Leftrightarrow$ `X. 5/4 = 6/7`
$\Leftrightarrow$ `X = 6/7 : 5/4`
$\Leftrightarrow$ `X = 24/35`
Vậy `X = 24/35`
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$2$`1/7` `-` `1``2/5` `=` `15/7 - 7/5 = 26/35`
`2``1/5` `+` `1``1/4 = 11/5 + 5/4 = 69/20`
Dễ thấy: `69/20 > 26/35`
`->` $2$`1/7` `-` `1``2/5 < 2``1/5` `+` `1``1/4`
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`4``1/15` x `3``1/3 = 61/15` x `10/3 = 122/9`
`3``4/7` : `2``7/9` `= 21/4 : 25/9 = 189/100`
Dễ thấy: `122/9 > 189/100`
`->` `4``1/15` x `3``1/3>` `3``4/7` : `2``7/9`
