Alg 2/trig

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I am doing Geometric sequences, but I cannot figure out how to find the Nth term of a geometric sequence.

The problem is A1 = 64, Ak+1 = 1/2Ak
(That is supposed to be A sub one, A sub k + 1, etc.)

The formula for finding the equation is:

An = A1 x R^n-1

The answer is:

An = 128(1/2)^n

I cannot figure out what to do and what happens to the n-1 and how it gets to just n.

Help is greatly appreciated. Thanks.

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