Question

4. Dado a PA (3, 7, 11, 15, ... ), a 2
soma dos 25 primeiros termos
dessa sequência é:
О 99
O 102
O 1275
O 1.450​

Answer 1

Explicação passo-a-passo:

• $\sf r=7-3$

$\sf r=4$

• $\sf a_n=a_1+(n-1)\cdot r$

$\sf a_{25}=a_1+24r$

$\sf a_{25}=3+24\cdot4$

$\sf a_{25}=3+96$

$\sf a_{25}=99$

• $\sf S_n=\dfrac{(a_1+a_n)\cdot n}{2}$

$\sf S_{25}=\dfrac{(3+99)\cdot25}{2}$

$\sf S_{25}=\dfrac{102\cdot25}{2}$

$\sf S_{25}=\dfrac{2550}{2}$

$\sf S_{25}=1275$