Sum of Geometric Sequence/Proof 4
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Theorem
Let $x$ be an element of one of the standard number fields: $\Q, \R, \C$ such that $x \ne 1$.
Let $n \in \N_{>0}$.
Then:
- $\ds \sum_{j \mathop = 0}^{n - 1} x^j = \frac {x^n - 1} {x - 1}$
Proof
Lemma
Let $n \in \N_{>0}$.
Then:
- $\ds \paren {1 - x} \sum_{i \mathop = 0}^{n - 1} x^i = 1 - x^n$
$\Box$
Then by the lemma:
\(\ds \paren {1 - x} \sum_{i \mathop = 0}^{n - 1} x^i\) | \(=\) | \(\ds 1 - x^n\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds \sum_{i \mathop = 0}^{n - 1} x^i = \frac {1 - x^n} {1 - x}\) | \(=\) | \(\ds \frac {x^n - 1} {x - 1}\) |
$\blacksquare$