A sequence of real numbers x1,x2,x3,…x_1, x_2, x_3, \dots satisfies the recurrence relation xn+1=5xn+22xn+3 for all n≥1.x_{n+1} = \frac{5x_n + 2}{2x_n + 3} \text{ for all } n \ge 1.

  1. (a)

    Given that the sequence converges to ll, find the possible exact values of ll.

  2. (b)

    Describe how the sequence behaves when x1=3x_1 = 3.

  3. (c)

    Given that x5=35032158x_5 = \frac{3503}{2158}, find the value of x1x_1.

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Worked solution

4(a)

1. Set up the equation for the limit $l$ by applying the convergence property.

As n→∞n \to \infty, xn→lx_n \to l and xn+1→lx_{n+1} \to l.
l=5l+22l+3l = \frac{5l + 2}{2l + 3}

2. Rearrange the equation into a standard quadratic form.

l(2l+3)=5l+2l(2l + 3) = 5l + 2
2l2+3l=5l+22l^2 + 3l = 5l + 2
2l2−2l−2=02l^2 - 2l - 2 = 0
l2−l−1=0l^2 - l - 1 = 0

3. Solve the quadratic equation for exact values of $l$.

l=−(−1)±(−1)2−4(1)(−1)2(1)l = \frac{-(-1) \pm \sqrt{(-1)^2 - 4(1)(-1)}}{2(1)}
l=1±1+42l = \frac{1 \pm \sqrt{1 + 4}}{2}
l=1±52l = \frac{1 \pm \sqrt{5}}{2}

Answer

l=1±52l = \frac{1 \pm \sqrt{5}}{2}

4(b)

1. Generate the first few terms of the sequence using the given recurrence relation and $x_1 = 3$.

Using a calculator:
x1=3x_1 = 3
x2=5(3)+22(3)+3=179≈1.889x_2 = \frac{5(3) + 2}{2(3) + 3} = \frac{17}{9} \approx 1.889
x3=5(17/9)+22(17/9)+3=10361≈1.689x_3 = \frac{5(17/9) + 2}{2(17/9) + 3} = \frac{103}{61} \approx 1.689
x4=5(103/61)+22(103/61)+3=637389≈1.638x_4 = \frac{5(103/61) + 2}{2(103/61) + 3} = \frac{637}{389} \approx 1.638

2. Determine the behaviour of the sequence based on the calculated terms.

Observing the values, x1>x2>x3>x4x_1 > x_2 > x_3 > x_4. The terms are strictly decreasing and approaching approximately 1.6181.618, which corresponds to the exact limit l=1+52l = \frac{1+\sqrt{5}}{2} found from solving l=5l+22l+3l = \frac{5l + 2}{2l + 3} in part (a).
Therefore, the sequence decreases and converges to 1+52\frac{1+\sqrt{5}}{2}.

Answer

The sequence decreases and converges to 1+52\frac{1+\sqrt{5}}{2}.

4(c)

1. Rearrange the given recurrence relation to make $x_n$ the subject.

xn+1=5xn+22xn+3x_{n+1} = \frac{5x_n + 2}{2x_n + 3}
xn+1(2xn+3)=5xn+2x_{n+1}(2x_n + 3) = 5x_n + 2
2xnxn+1+3xn+1=5xn+22x_n x_{n+1} + 3x_{n+1} = 5x_n + 2
2xnxn+1−5xn=2−3xn+12x_n x_{n+1} - 5x_n = 2 - 3x_{n+1}
xn(2xn+1−5)=2−3xn+1x_n(2x_{n+1} - 5) = 2 - 3x_{n+1}
xn=2−3xn+12xn+1−5x_n = \frac{2 - 3x_{n+1}}{2x_{n+1} - 5}

2. Use the inverse recurrence relation iteratively to compute $x_4, x_3, x_2$, and $x_1$ backwards from $x_5$.

Given x5=35032158x_5 = \frac{3503}{2158}, using a graphing calculator to compute the previous terms iteratively:
x4=2−3(35032158)2(35032158)−5=563344x_4 = \frac{2 - 3\left(\frac{3503}{2158}\right)}{2\left(\frac{3503}{2158}\right) - 5} = \frac{563}{344}
x3=2−3(563344)2(563344)−5=9154x_3 = \frac{2 - 3\left(\frac{563}{344}\right)}{2\left(\frac{563}{344}\right) - 5} = \frac{91}{54}
x2=2−3(9154)2(9154)−5=158x_2 = \frac{2 - 3\left(\frac{91}{54}\right)}{2\left(\frac{91}{54}\right) - 5} = \frac{15}{8}
x1=2−3(158)2(158)−5=2910x_1 = \frac{2 - 3\left(\frac{15}{8}\right)}{2\left(\frac{15}{8}\right) - 5} = \frac{29}{10}

Answer

2910\frac{29}{10}

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