1.5.1 What is the sum of the geometric series \( S_{n}=a+a r+a r^{2}+\ldots a r^{n-1} \) if \( r=-1 \) ?
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Bonus Knowledge
When the common ratio \( r = -1 \), the terms of the geometric series alternate between \( a \) and \( -a \). The series can be expressed as \( S_n = a - a + a - a + \ldots \). For even \( n \), the sum \( S_n \) will equal \( 0 \) as the positive and negative terms cancel each other out. However, for odd \( n \), there will be one extra \( a \), making the sum \( S_n = a \). For example, if \( n = 5 \) (which is odd), the series would look like \( a - a + a - a + a = a \). Conversely, for \( n = 4 \) (even), it turns into \( a - a + a - a = 0 \). Thus, the final answer depends on \( n \) being odd or even!
