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Example
Determine the values of r so that the sequence
is convergent.
Complete Solution
We can solve this problem by considering cases for the value of r.
Case 1
If r > 1, then rn tends to infinity as n tends to infinity. The sequence is divergent in this case.
Case 2
If r= 1, then
so the sequence is convergent for this case.
Case 3
If , then
so the sequence is convergent for this case.
Case 4
If , then
does not exist, so the sequence is divergent for this case.
Case 5
If , then tends to negative infinity as does not tend to a single finite number. The sequence is divergent in this case.
Summary
Therefore, the sequence is convergent when .
Explanation of Each Step
Case 1
Consider the case when . Then our sequence becomes
which tends to infinity.
Case 2
Here we are using a fundamental property of limits, that the limit of a constant equals that constant:
for any constant and .
Case 3
Consider the case when . Then our sequence becomes
which tends to zero.
Similarly, if . Then our sequence becomes
which also tends to zero.
Case 4
In this case, we have the sequence
As approaches infinity the sequence does not approach a unique value, so the limit does not exist.
Case 5
This case is similar to Case 1. Consider the case when . Then our sequence becomes
The terms alternate between positive and negative numbers, and do not tend to a single finite number.
Possible Challenge Areas
Connecting Results to Definition of Convergence
In each of the cases, we used a limit to determine whether the sequence is convergent. According to our definition of convergence of a sequence, as long as our respective limits exist, then the sequence converges.
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