c) x -1/2 -1/6 -1/12 -1/20 = 1/30

* Hướng dẫn giải

$\begin{array}{l}\mathrm{c}) \mathrm{x}-\frac{1}{2}-\frac{1}{6}-\frac{1}{12}-\frac{1}{20}=\frac{1}{30}\\ \mathrm{x}=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}\\ \mathrm{x}=\frac{1}{2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}\\ \mathrm{x}=\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\\ \mathrm{x}=\frac{1}{2}+\frac{1}{2}-\frac{1}{6}\\ \mathrm{x}=1-\frac{1}{6}\\ \mathrm{x}=\frac{5}{6}\\ \mathrm{Vậy} \mathrm{x}=\frac{5}{6}\end{array}$