A sequence of real numbers satisfies the recurrence relation
- (a)
Given that the sequence converges to , find the possible exact values of .
- (b)
Describe how the sequence behaves when .
- (c)
Given that , find the value of .
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Worked solution
4(a)
1. Set up the equation for the convergence limit.
Since the sequence converges to , we can take the limit as on both sides of the recurrence relation, setting and :
2. Rearrange the equation into a standard quadratic form.
3. Solve the quadratic equation to find the exact values of $l$.
Using the quadratic formula:
4(b)
1. Calculate the first few terms to observe the sequence's behaviour.
The values are strictly decreasing.
2. Determine the value the sequence converges to by observing long-term behaviour.
As , using the calculator to evaluate subsequent terms shows that approaches approximately , which corresponds to (the positive possible exact value of found in part (a)).
Therefore, the sequence decreases and converges to .
The sequence decreases and converges to .
4(c)
1. Make $x_n$ the subject of the recurrence relation.
2. Iterate backwards from $x_5$ to find $x_1$ using a graphing calculator.
Using a graphing calculator to generate the previous terms sequentially:
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