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第68页

信息发布者：

$\frac {7}{12}$

$\frac 45$

$\frac 15$

$\begin{aligned}解：x&=\frac {3}{5}-\frac {5}{9}\\x&=\frac {2}{45}\end{aligned}$

$\begin{aligned}解：x&=\frac {3}{8}+\frac {5}{12}\\x&=\frac {19}{24}\end{aligned}$

$\begin{aligned}解：x&=\frac {1}{6}+\frac {5}{12}\\x&=\frac {7}{12}\end{aligned}$

$\begin{aligned}解：x&=\frac {8}{9}-\frac {1}{3}\\x&=\frac {5}{9}\end{aligned}$

$\frac{13}{42}$

$\frac{20}{99}$

$\frac{23}{120}$

$\frac{1}{42}$

$\frac{2}{99}$

$\frac{7}{120}$

积

和或差

$\frac{17}{72}$

$\frac{1}{56}$

$\frac{12}{35}$

$\frac{3}{130}$

$\frac{17}{60}$

$\frac{7}{144}$

$\frac 12+\frac 38+\frac {1}{10}=\frac {39}{40}$

$\frac {39}{40}＜1$

答：五年级一班同学本学期没有都阅读课外书。

$\frac 32$

$=\frac 12+(\frac 13+\frac 23)+(\frac 14+\frac 24+\frac 34)+…+(\frac {1}{12}+\frac {2}{12}+…+\frac {11}{12})$

$=\frac 12+\frac 22+\frac 32+…+\frac {11}{2}$

$=\frac {1+2+3…+11}2$

$=33$