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$\frac{7}{12}$

$\frac{13}{36}$

$\frac{12}{35}$

$\frac{17}{72}$

$\frac{1}{12}$

$\frac{5}{36}$

$\frac{2}{35}$

$\frac{1}{72}$

?$1-\frac {5}{9}$?、?$ \frac {5}{7}-\frac {1}{6}$?、?$ \frac {1}{5}+\frac {3}{8}$?接近?$ \frac 12$?。

?$=\frac {9}{90}+\frac {70}{90}$?

?$=\frac {79}{90}$?

?$=\frac 99-\frac 59$?

?$=\frac {4}{9}$?

?$=\frac {30}{42}-\frac {7}{42}$?

?$=\frac {23}{42}$?

?$=\frac {8}{40}+\frac {15}{40}$?

?$=\frac {23}{40}$?

+

-

+

$??\begin{aligned} 解： x&=\frac 56-\frac 12\\x&=\frac 13\end{aligned}??$

$??\begin{aligned} 解： x&=\frac 12-\frac 14\\x&=\frac 14\end{aligned}??$

$??\begin{aligned} 解： x&=\frac 23+\frac 14\\x&=\frac {11}{12}\end{aligned}??$

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