=> x- (\(\frac{20}{11.13}\) + \(\frac{20}{13.15}\) +...+ \(\frac{20}{53.55}\)) = \(\frac{3}{11}\)
=> x - 10.(\(\frac{2}{11.13}\) + \(\frac{2}{13.15}\) +...+ \(\frac{2}{53.55}\)) = \(\frac{3}{11}\)
=> x - 10.( \(\frac{1}{11}\) - \(\frac{1}{13}\) + \(\frac{1}{13}\) - \(\frac{1}{15}\) +...+ \(\frac{1}{53}\) - \(\frac{1}{55}\)) = \(\frac{3}{11}\)
=> x - 10. (\(\frac{1}{11}\) - \(\frac{1}{55}\)) = \(\frac{3}{11}\)
=> x - 10.\(\frac{4}{55}\) = \(\frac{3}{11}\)
=> x - \(\frac{8}{11}\) = \(\frac{3}{11}\)=> x=1 Vậy x=1
Ta có
\(Đăt\) \(M=-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}\)
\(=10\left(\frac{2}{11.13}-\frac{2}{13.15}-...-\frac{2}{53.55}\right)\)
\(=10\left(-\frac{1}{11}+\frac{1}{13}-\frac{1}{13}+\frac{1}{15}-...-\frac{1}{53}+\frac{1}{55}\right)\)
\(=10\left(-\frac{1}{11}+\frac{1}{55}\right)=\frac{-40}{55}\)
Thê M vào bt có
\(x-\frac{-40}{55}=\frac{3}{11}\)
còn lại tự giải