B1:

`B` = $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + ... +$\frac{1}{72}$

`B` = $\frac{1}{2 x 3}$ + $\frac{1}{3 x 4}$ + $\frac{1}{4 x 5}$ + $\frac{1}{5 x 6}$ + ... +$\frac{1}{8 x 9}$

`B` = $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + ... + $\frac{1}{8}$ - $\frac{1}{9}$

`B` = $\frac{1}{2}$ - $\frac{1}{9}$

`B` = $\frac{7}{18}$

B2: Tìm `x`

`C` = ( $\frac{1}{42}$ + $\frac{1}{56}$ + ... + $\frac{1}{90}$ ) x `x` = `1`

`C` = (  $\frac{1}{6 x 7}$ + $\frac{1}{7 x 8}$ + ... +$\frac{1}{9 x 10}$ ) x `x` = `1`

`C` = ( $\frac{1}{6}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{8}$ + ... + $\frac{1}{9}$ - $\frac{1}{10}$ ) x `x` = `1`

`C` = ( $\frac{1}{6}$ - $\frac{1}{10}$) x `x` = `1`

`C` = $\frac{1}{15}$ x `x` = `1`

`C` = `x` = `1` : $\frac{1}{15}$

`C` = `x` = `1`.$\frac{15}{1}$

`C` = `x` = `15`

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