Question

1-determine a soma dos 18 primeiros termos da P.A.(1,4,7...)

Answer 1

a1 = 1
a2 = 4
a3 = 7
n = 18
r = 4 - 1 = 3 ***
a18 = 1 + 17(3) = 1 + 51 = 52 ***

S18 = ( 1 + 52) * 9
s18 = 53 * 9 = 477****

Answer 2

$\sf a_n=a_1+(n-1)\cdot r$

$\sf a_{18}=1+(18-1)\cdot3$

$\sf a_{18}=1+17\cdot3$

$\sf a_{18}= 1+51$

$\sf a_{18}=52$

$\sf S_{18}=(a_1+a_{18})\cdot(18/2)$

$\sf S_{18}=(1+52)\cdot(9)$

$\sf S_{18}=53\cdot9$

$\red{\boldsymbol{S_{18}=477}}$