Question

Numa PA de n termos e razão r temos a1=-2/15, an=2/3 e rn=1. Calcule r e n,

Answer 1

Boa tarde

a1 = -2/15 
an = 2/3
r*n = 1 

termo geral
an = a1 + r*(n - 1)

2/3 = -2/15 + (n - 1)/n 

(n - 1)/n = 10/15 + 2/15 = 12/15 = 4/5

5*(n - 1) = 4n
5n - 5 = 4n
5n - 5n = 5

n = 5 
r = 1/n = 1/5