[tex]\dfrac{1}{1\cdot3}+\dfrac{1}{3\cdot5}+...+\dfrac{1}{2013\cdot2014}=\dfrac12\cdot\left(\dfrac11-\dfrac13+\dfrac13-\dfrac15+\dfrac15-\dfrac17+...[/tex]
[tex]...+\dfrac{1}{2013}-\dfrac{1}{2015})\right)=\dfrac12\cdot\dfrac{2014}{2015}=\dfrac{1012}{2015}[/tex]
Inegalitatea din enunt devine acum
[tex]\dfrac25<\dfrac{1012}{2015}<\dfrac12[/tex]
care adusa la acelasi numarator devine:
[tex]\dfrac{1012}{2530}<\dfrac{1012}{2015}<\dfrac{1012}{2014}[/tex] care este adevarata (dintre doua fractii cu acelasi numarator este mai mare cea cu numitorul mai mic.)