\(E=\frac{2}{10x12}+\frac{2}{12x14}+...+\frac{2}{108x110}\)
\(\Rightarrow E=\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}+...+\frac{1}{108}-\frac{1}{110}\)
\(\Rightarrow E=\frac{1}{10}-\frac{1}{110}=\frac{10}{110}=\frac{1}{11}\)
Công thức :\(\frac{a}{n\left(n+a\right)}=\frac{1}{n}-\frac{1}{n+a}\)
\(E=\frac{2}{10\cdot12}+\frac{2}{12\cdot14}+\frac{2}{14\cdot16}+...+\frac{2}{108\cdot110}\)
\(E=\frac{1}{10\cdot12}+\frac{1}{12\cdot14}+\frac{1}{14\cdot16}+...+\frac{1}{108\cdot110}\)
\(E=\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}+\frac{1}{14}-\frac{1}{16}+...+\frac{1}{108}-\frac{1}{110}\)
\(E=\frac{1}{10}-\frac{1}{110}\)
\(E=\frac{11}{110}-\frac{1}{110}\)
\(E=\frac{10}{110}\)\(=\frac{1}{11}\)
Học tốt nha bạn !!!
