\(\left(\frac{2}{11.13}+\frac{2}{13.15}+...+\frac{2}{19.21}\right)-x+\frac{221}{231}=\frac{4}{3}\)
Ta có:
\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+....+\frac{1}{19}-\frac{1}{21}\right)-x+\frac{221}{231}=\frac{4}{3}\)
\(\left(\frac{1}{11}-\frac{1}{21}\right)-x+\frac{221}{231}=\frac{4}{3}\)
\(\frac{10}{231}-x+\frac{221}{231}=\frac{4}{3}\)
\(\frac{10}{231}-x=\frac{4}{3}-\frac{221}{231}=\frac{29}{77}\)
\(x=\frac{10}{231}-\frac{29}{77}\)
\(x=-\frac{1}{3}\)