[tex] \frac{1}{n}- \frac{1}{n+1} = \\ \text{Numitorul comun este: }\;\;\;n(n+1) \\ = \frac{n+1}{n(n+1)}- \frac{n}{n(n+1)} = \frac{n+1-n}{n(n+1)} = \frac{1}{n(n+1)}[/tex]
[tex] \frac{1}{n}- \frac{1}{n+3} = \\ \text{Numitorul comun este: }\;\;\;n(n+3) \\ = \frac{n+3}{n(n+3)}- \frac{n}{n(n+3)}= \frac{n+3-n}{n(n+3)}= \frac{3}{n(n+3)} [/tex]