Respuesta :

meerkat18
For geometric sequence, the general formula is:

An = A₀rⁿ⁻¹

So for n=3 and An = 13
13 = A₀r² 
A₀ = 13/r² --> eqn 1

For n=7 and An = 208
208 = A₀r⁶ --> eqn 2

Solving simultaneously,

208 = (13/r²)(r⁶)
208 = 13r⁴
r = ⁴√208/13 = 2

Then, A₀ = 13/2² = 13/4

1. The general formula would then be: An = (13/4)(2)ⁿ⁻¹
2. If n=19,
An = (13/4)(2¹⁸) = 851,968

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