Question

4. Dado a PA (3, 7, 11, 15, ...), a 2 pontos
soma dos 25 primeiros termos
dessa sequência é: *

99
102
1275
1 450​

Answer 1

Explicação passo-a-passo:

• $\sf r=7-3$

$\sf r=4$

• $\sf a_n=a_1+(n-1)\cdot r$

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

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

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

$\sf a_{25}=99$

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

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

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

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

$\sf S_{25}=1275$