Question

calcule a soma dos 22 primeiros termos de PA (80,75,70...)

Answer 1

$a_n=a_1+(n-1).r$
$a_{22}=80+(22-1).(-5)$          $r=a_2-a_1=75-80=-5$
$a_{22}=80+21.(-5)$
$a_{22}=80-105$
$a_{22}=-25$

$S_n= \frac{(a_1+a_n).n}{2}$
$S_{22}= \frac{[80+(-25)].22}{2}$
$S_{22}=[80-25].11$
$S_{22}=55.11$
$S_{22}=605$

Answer 2

resolução!

r = a2 - a1

r = 75 - 80

r = - 5

a22 = a1 + 21r

a22 = 80 + 21 * (-5)

a22 = 80 + (-105)

a22 = - 25

Sn = ( a1 + an ) n / 2

Sn = ( 80 + (-25 ) 22 / 2

Sn = 55 * 11

Sn = 605