Ta có M - N = 1,8 và M x 6 - N = 12,8 => 5M = 12,8 - 1,8 5M = 11 M = 11 : 5 M = 2,2 => N = 2,2 - 1,8 N = 0,4 Vậy M = 2,2 ; N = 0,4

= 2 x( \(\dfrac{2}{1\text{x}3}\)+\(\dfrac{2}{3\text{x}5}\)+\(\dfrac{2}{5\text{x}7}\)+\(\dfrac{2}{7\text{x}9}\)+...+\(\dfrac{2}{15\text{x}17}\)+\(\dfrac{2}{17\text{x}19}\)+\(\dfrac{2}{19\text{x}21}\)) = 2 x ( \(\dfrac{1}{1}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{7}\) +...+\(\dfrac{1}{17}\) - \(\dfrac{1}{19}\) + \(\dfrac{1}{19}\) - \(\dfrac{1}{21}\) ) = 2 x ( 1 - \(\dfrac{1}{21}\) ) = 2 x \(\dfrac{20}{21}\) = \(\dfrac{40}{21}\)