Question

numa p.g (1,2,4,..256),calcule n

Answer 1

               (n - 1)
An = a1.q
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                 (n - 1)
256 = 1 . 2
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 (n - 1)         8
2         =    2
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n - 1 = 8
n = 8 + 1
n = 9

Answer 2

PG(1,2,4,...,256)

a1 = 1
a2 = 2
an = 256

q = a2/a1 = 2/1 = 2

an = a1.q^(n-1)

256 = 1.2^(n-1)
2^8 = 2^(n-1)

n-1 = 8
n = 8 + 1
n = 9

Espero ter ajudado.