Question

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

Answer 1

(5,20,....1280)

an=5.4^n-1

1280/5=4^n-1

256=2^2n-2

2^8=2^2n-2

8=2n-2

8+2=2n
10=2n

n=10/2=5

S5=5.(4^5-1)/4-1

S5=5(1024-1)/3

S5=5.(1023)/3

S5=5115/3

S5=1705