Ptrinh tương đương vs
$2(\dfrac{1}{2.3} + \dfrac{1}{3.4} + \cdots + \dfrac{1}{x(x+1)}) = \dfrac{2007}{2009}$
$<-> \dfrac{1}{2} - \dfrac{1}{3} + \dfrac{1}{3} - \dfrac{1}{4} + \cdots + \dfrac{1}{x} - \dfrac{1}{x+1} = \dfrac{2007}{4018}$
$<-> \dfrac{1}{2} - \dfrac{1}{x+1} = \dfrac{2007}{4018}$
$<-> \dfrac{1}{x+1} = \dfrac{1}{2} - \dfrac{2007}{4018}$
$<-> \dfrac{1}{x+1} = \dfrac{1}{2009}$
$<-> x+1 = 2009$
$<-> x = 2008$.
Vậy nghiệm là 2008.
