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Worked solution

Use a logarithmic fixed-point graph to compare terms

The diagram shows y=ln⁡(x+2)−x/5y=\ln(x+2)-x/5. Its positive root is α\alpha. Find α\alpha. Positive terms satisfy xn+1=5ln⁡(xn+2)x_{n+1}=5\ln(x_n+2).

(i) Prove algebraically that convergence implies limit α\alpha.

(ii) Use a calculator to describe the behaviour for x1=1x_1=1 and x1=20x_1=20.

(iii) By considering xn+1−xnx_{n+1}-x_n, prove increase when 0<xn<α0<x_n<\alpha and decrease when xn>αx_n>\alpha.

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Graph h(x)=ln(x+2) minus x/5, with vertical asymptote x=minus 2 and positive root alpha approximately 13.8001. It is positive for 0<x<alpha and negative for x>alpha.
Graph h(x)=ln(x+2) minus x/5, with vertical asymptote x=minus 2 and positive root alpha approximately 13.8001. It is positive for 0<x<alpha and negative for x>alpha.
Graph h(x)=ln(x+2) minus x/5, with vertical asymptote x=minus 2 and positive root alpha approximately 13.8001. It is positive for 0<x<alpha and negative for x>alpha.

[2008/IJC/MYE/12(modified)]

CJC · Discussion · Question 15

Answer

α≈13.8\alpha\approx13.8 (3 s.f.); increasing from 11, decreasing from 2020, both towards α\alpha.

Locate the positive root

Solving ln⁡(x+2)−x/5=0\ln(x+2)-x/5=0 numerically gives α≈13.80007271\alpha\approx13.80007271, or 13.813.8 to 3 significant figures. With h(x)=ln⁡(x+2)−x/5h(x)=\ln(x+2)-x/5, h(0)>0h(0)>0, h′(x)=1/(x+2)−1/5h^\prime(x)=1/(x+2)-1/5 changes sign at 33, and h(x)→−∞h(x)\to-\infty. Thus there is exactly one positive root.

How did this part go?

(i) Pass to the limit algebraically

If the positive sequence converges to ℓ≥0\ell\ge0, then ℓ=5ln⁡(ℓ+2)\ell=5\ln(\ell+2), so h(ℓ)=0h(\ell)=0. Since h(0)>0h(0)>0 and the positive root is unique, ℓ=α\ell=\alpha.

How did this part go?

(ii) Retain the calculator observations

For x1=1x_1=1, the printed selected terms are:

nxₙ
11
25.49
310.07
613.7
713.8
813.8

The terms increase towards α\alpha. For x1=20x_1=20:

nxₙ
120
215.5
314.3
414.0
513.8
613.8

The terms decrease towards α\alpha. Rounded repetitions of 13.8 are not exact equality.

The calculator screenshots also show all eleven rows at their displayed precision. Enter the same recurrence in sequence mode and change only the initial value between the two runs:

nxₙ from x₁=1xₙ from x₁=20
1120
25.493115.455
310.0714.298
412.45413.955
513.35513.849
613.65713.816
713.75513.805
813.78613.802
913.79613.801
1013.79913.8
1113.813.8

The shorter printed tables above use different rounding from the calculator screenshots. Both display versions are retained; these rounded repetitions do not mean the recurrence has reached its exact fixed point.

How did this part go?

(iii) Use the sign of the increment

xn+1−xn=5h(xn)x_{n+1}-x_n=5h(x_n). For 0<xn<α0<x_n<\alpha, h(xn)>0h(x_n)>0, giving xn+1>xnx_{n+1}>x_n; for xn>αx_n>\alpha, h(xn)<0h(x_n)<0, giving xn+1<xnx_{n+1}<x_n. Since 5ln⁡(x+2)5\ln(x+2) is increasing and fixes α\alpha, a term below α\alpha stays below it, and a term above stays above. The two observed sequences are therefore monotone and bounded and converge to α\alpha.

How did this part go?

Compare your sketch

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Increment graph g(x)=5 ln(x+2) minus x. For positive x it is above the axis until alpha approximately 13.8001, then below the axis. Vertical asymptote at x=minus 2.
Increment graph g(x)=5 ln(x+2) minus x. For positive x it is above the axis until alpha approximately 13.8001, then below the axis. Vertical asymptote at x=minus 2.
Increment graph g(x)=5 ln(x+2) minus x. For positive x it is above the axis until alpha approximately 13.8001, then below the axis. Vertical asymptote at x=minus 2.

Common mistakes

Rounded calculator terms can repeat before the sequence actually reaches its limit.

Read the hint