Răspuns:
S = 14
Explicație pas cu pas:
[tex]\boxed{ \bf a_{n} = a_{1} + (n - 1) \cdot r}[/tex]
[tex]a_{3} = a_{1} + 2 \\ 4 = 2 + 2 \cdot r \iff 2 \cdot r = 2 \\ \implies \bf r = 1[/tex]
[tex]a_{4} = a_{1} + 3 \cdot r = 2 + 3 \cdot 1 = 2 + 3 = 5 \\ [/tex]
[tex] \boxed{ \bf S_{n} = \frac{(a_{1} + a_{n})\cdot n}{2}}[/tex]
[tex]S_{4} = \frac{(a_{1} + a_{4})\cdot 4}{2} = \frac{(2 + 5)\cdot 4}{2} = 7\cdot 2 = 14 \\ [/tex]
