# How do you find the sum of a finite geometric sequence from n = 1 to n = 8, using the expression #−2(3)^n − 1#?

##### 1 Answer

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#### Explanation

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I could just use the formula, plug in the values and get the answer. Not going to do that!

The sum is

#### Explanation:

So that we can link this to the generic formula bit let the geometric ratio of 3 be

Then we have:

but

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Set

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Thus by substitution in

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Check:

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