Question

determine a soma dos termos da pg finita (5,20,...,1280)

Answer 1

Ola Cintia 

PG 

u1 = 5
u2 = 20

q = u2/u1 = 20/5 = 4

termo geral

un = u1*q^(n - 1) 

1280 = 5*4^(n - 1)

4^(n - 1) = 1280/5 = 256 = 4^4

n - 1 = 4

n = 5 

soma
Sn = u1*(q^n - 1)/(q - 1) 

Sn = 5*(4^5 - 1)/(4 - 1) 

Sn = 5*1023/3 =  1705 

.