To prove: The formula for the sum of a finite geometric serious with first term a and common ratio , .
If are real numbers and m and n are integers such that , then .
By the above definition, the expression simplified as follows.
That is, (1)
Multiply both side of the equation (2) by r gives,
That is, (3)
Subtract equation (2) from the equation (3).
That is, (4)
Compare the equation (1) with equation (4).
Hence the proof.
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