X - 20/11 . 13 - 20/13 . 15 - ...- 20/53 . 55 = 25/11
X - ( 20/11 . 13 + 20/13 . 15 + ...+ 20/53 . 55 ) = 25/11
X - 10 . ( 2/11 . 13 + 2/13 . 15 + ...+ 2/53 . 55 ) = 25/11
X - 10. ( 1/11 - 1/13 + 1/13 - 1/15 + ...+ 1/53 - 1/55 ) = 25/11
X - 10. ( 1/11 - 1/55 ) = 25/11
X - 10 . 4/55 = 25/11
X - 8/11 = 25/11
X = 25/11 + 8/11
X = 33/11
X = 3
Tham khảo nha !!!
\(x-\frac{20}{11\times13}-\frac{20}{13\times15}-...-\frac{20}{53\times5}=\frac{25}{11}\)
\(\Rightarrow x-\frac{\left(\frac{5}{11}+\frac{5}{13}-\frac{5}{13}+\frac{5}{15}-\frac{5}{15}-...-\frac{5}{53}+\frac{5}{55}\right)}{6}=\frac{25}{11}\)
\(\Rightarrow x-\frac{\left(\frac{5}{11}+\frac{5}{55}\right)}{6}=\frac{25}{11}\)
\(\Rightarrow x=\frac{25}{11}+\frac{\left(\frac{5}{11}+\frac{5}{55}\right)}{6}\)
\(\Rightarrow x=\frac{26}{11}\)
