Question

Dada a PA (1,3,5,7...)
Qual a soma dos 20 primeiros termos?

Answer 1

1-3=2. r=2
An=a¹+(n-1)•r
an=1+(20-1)•2
an=1+19•2
an=1+38
an=39
sn=(a¹+an)•n/2
sn=(1+39)•20/2
sn=40•20/2
sn=800/2
sn=400
ta certinho!

Answer 2

Formula: an = a1 + (n - 1).r

an = ?
a1 = 1
n = 20
r = 2


an = 1 + (20 - 1).2
an = 1 + 19.2
an = 1 + 38
an = 39


Sn = (a1 + an).n/2


Sn = (1 + 39).20/2
Sn = 40.20/2
Sn = 800/2
Sn = 400


A soma dos 20 primeiros termos é 400.