Tính hợp lí: A= 1/2 + 1/6 + 1/12 + 1/30

* Hướng dẫn giải

$a)A=\frac{5}{6}$

$\begin{array}{l}b)B=\frac{10}{39}\\ \\ c)C=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+\frac{1}{11.13}\\ \\ =\frac{1}{2}\left(\frac{1}{3}-\frac{1}{13}\right)=\frac{5}{39}\end{array}$