Explicație pas cu pas:
[tex]a_{1} = 5[/tex]
7 - 5 = 2
9 - 7 = 2
=> r = 2
[tex]\boxed {a_{n} = a_{1} + (n- 1)\cdot r}[/tex]
[tex]a_{5} = 5 + (5- 1)\cdot 2 = 5 + 4\cdot 2 = 5 + 8 = \bf 13 \\ [/tex]
[tex]\boxed {S_{n} = \frac{2\cdot a_{1} + (n - 1)\cdot r}{2} \cdot n} \\ [/tex]
[tex]S_{5} = \frac{2\cdot5 + (5 - 1)\cdot 2}{2} \cdot 5 = \\ = \frac{10 + 8}{2} \cdot 5 = 9 \cdot 5 = \bf45[/tex]
