a/$\frac{2}{3.5}$ + $\frac{2}{7.9}$ +…+ $\frac{2}{99.101}$ b/ $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$

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1. $a$) $\dfrac{2}{3.5} + \dfrac{2}{5.7} + \dfrac{2}{7.9} + ….. + \dfrac{2}{99.101}$

   $ = \dfrac{1}{3} – \dfrac{1}{5} + \dfrac{1}{5} – \dfrac{1}{7} + \dfrac{1}{7} – \dfrac{1}{9} + ….. + \dfrac{1}{99} – \dfrac{1}{101}$

   $= \dfrac{1}{3} – \dfrac{1}{101}$

   $= \dfrac{98}{303}$

   $b$) $\dfrac{1}{20} + \dfrac{1}{30} + \dfrac{1}{42} + \dfrac{1}{56} + \dfrac{1}{72} + \dfrac{1}{90}$

   $= \dfrac{1}{4.5} + \dfrac{1}{5.6} + \dfrac{1}{6.7} + \dfrac{1}{7.8} + \dfrac{1}{8.9}  + \dfrac{1}{9.10}$

   $= \dfrac{1}{4} – \dfrac{1}{10}$

   $= \dfrac{3}{20}$

   $c$ ) $\dfrac{x+30}{2} = \dfrac{x-29}{3}$

   $⇔ \dfrac{3x+90}{6} = \dfrac{2x-58}{6}$

   $⇔ 3x + 90 = 2x – 58$

   $⇔ 90 + 58 = 2x – 3x$

   $⇔ 148 = -x$

   $⇔ x = -148$

    Vậy $x=-148$

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