Tính hợp lí: A=1/56+1/72+1/90+1/110+1/132+1/156

* Hướng dẫn giải

$a)A=\frac{1}{7}-\frac{1}{13}=\frac{6}{91}.$

$\begin{array}{l}b)B=\frac{1}{3}-\frac{1}{27}=\frac{8}{27}\\ \\ c)C= \frac{1}{4} \left(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+\frac{4}{15.19}+\frac{4}{19.23}+\frac{4}{23.27}\right)=\frac{2}{27}\\ \\ d)D=\frac{1}{5}\left(1-\frac{1}{31}\right)=\frac{6}{31}\end{array}$