Question

qual a soma dessa P.A dada
( 5,8,1...)

Answer 1

a1 = 5
r = 8 
an = a1+(n-1).r
an = 5+(n-1).8
an = 5 + 8n - 8
an = 8n-3
Sn = (a1+an).n/2
Sn = (5+8n-3).n/2
Sn = (2+8n).n/2
Sn = (1+4n).n
Sn = 4n² + n