Question

Va rog sa mi explicati

Answer 1

Răspuns:

S = 14

Explicație pas cu pas:

$\boxed{ \bf a_{n} = a_{1} + (n - 1) \cdot r}$

$a_{3} = a_{1} + 2 \\ 4 = 2 + 2 \cdot r \iff 2 \cdot r = 2 \\ \implies \bf r = 1$

$a_{4} = a_{1} + 3 \cdot r = 2 + 3 \cdot 1 = 2 + 3 = 5$

$\boxed{ \bf S_{n} = \frac{(a_{1} + a_{n})\cdot n}{2}}$

$S_{4} = \frac{(a_{1} + a_{4})\cdot 4}{2} = \frac{(2 + 5)\cdot 4}{2} = 7\cdot 2 = 14$