Đặt :
\(M=\dfrac{1}{4}+\dfrac{1}{28}+\dfrac{1}{70}+=-+\dfrac{1}{550}\)
\(\Rightarrow M=\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.10}+=-..+\dfrac{1}{22.25}\)
\(\Rightarrow3M=\dfrac{3}{1.4}+\dfrac{3}{4.7}+=-.+\dfrac{3}{22.25}\)
\(\Rightarrow3M=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+--.+\dfrac{1}{22}-\dfrac{1}{25}\)
\(\Rightarrow3M=1-\dfrac{1}{25}\)
\(\Rightarrow3M=\dfrac{24}{25}\)
\(\Rightarrow M=\dfrac{24}{75}\)
