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Section 10.3 : Series - Basics

2. Perform an index shift so that the following series starts at \(n = 3\).

\[\sum\limits_{n = 7}^\infty {\frac{{4 - n}}{{{n^2} + 1}}} \] Show Solution

There really isn’t all that much to this problem. Just remember that, in this case, we’ll need to decrease the initial value of the index by four so it will start at \(n = 3\) and this means all the \(n\)’s in the series terms will need to increase by the same amount (four in this case…).

Doing this gives the following series.

\[\sum\limits_{n = 7}^\infty {\frac{{4 - n}}{{{n^2} + 1}}} = \sum\limits_{n = 3}^\infty {\frac{{4 - \left( {n + 4} \right)}}{{{{\left( {n + 4} \right)}^2} + 1}}} = \require{bbox} \bbox[2pt,border:1px solid black]{{\sum\limits_{n = 3}^\infty {\frac{{ - n}}{{{{\left( {n + 4} \right)}^2} + 1}}} }}\]

Be careful with parenthesis, exponents, coefficients and negative signs when “shifting” the \(n\)’s in the series terms. When replacing \(n\) with \(n + 4\) make sure to add in parenthesis where needed to preserve coefficients and minus signs.