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Algebra Difficulty 6.2 National Olympiad Prove it Philippines

Problem:
Let α\alpha be the unique positive root of the equation
x201811x24=0 x^{2018} - 11x - 24 = 0
Find α2018\left\lfloor \alpha^{2018} \right\rfloor. (Here, x\lfloor x\rfloor denotes the greatest integer less than or equal to xx.)

Solution

Solution:
Let α\alpha be the unique positive root of x201811x24=0x^{2018} - 11x - 24 = 0.

So,
α2018=11α+24 \alpha^{2018} = 11\alpha + 24
We are to find α2018\left\lfloor \alpha^{2018} \right\rfloor.

Let us estimate α\alpha.

Suppose α>1\alpha > 1. Try α=2\alpha = 2:
2201811×224=2201846 2^{2018} - 11 \times 2 - 24 = 2^{2018} - 46
which is very large and positive. Try α=1\alpha = 1:
1201811×124=11124=34 1^{2018} - 11 \times 1 - 24 = 1 - 11 - 24 = -34
So the root α\alpha is between 11 and 22.

Let us try α=1.01\alpha = 1.01:
(1.01)2018e2018×0.01=e20.185.8×108 (1.01)^{2018} \approx e^{2018 \times 0.01} = e^{20.18} \approx 5.8 \times 10^8
So α\alpha is much closer to 11 than 22.

But let's use the equation:
α2018=11α+24 \alpha^{2018} = 11\alpha + 24
Let A=α2018A = \alpha^{2018}. Then A=11α+24A = 11\alpha + 24.

But α\alpha is just slightly more than 11, so α=1+ε\alpha = 1 + \varepsilon for small ε>0\varepsilon > 0.

Then,
α2018=(1+ε)20181+2018ε \alpha^{2018} = (1 + \varepsilon)^{2018} \approx 1 + 2018\varepsilon
So,
1+2018ε=11(1+ε)+24=35+11ε 1 + 2018\varepsilon = 11(1 + \varepsilon) + 24 = 35 + 11\varepsilon
So,
1+2018ε35+11ε 1 + 2018\varepsilon \approx 35 + 11\varepsilon
2018ε11ε34 2018\varepsilon - 11\varepsilon \approx 34
2007ε34 2007\varepsilon \approx 34
ε342007 \varepsilon \approx \frac{34}{2007}
So,
α1+342007 \alpha \approx 1 + \frac{34}{2007}
Now,
α2018(1+342007)2018 \alpha^{2018} \approx (1 + \frac{34}{2007})^{2018}
Let x=3420070.01694x = \frac{34}{2007} \approx 0.01694.

Then,
(1+x)2018e2018x=e2018×0.01694=e34.26.6×1014 (1 + x)^{2018} \approx e^{2018x} = e^{2018 \times 0.01694} = e^{34.2} \approx 6.6 \times 10^{14}
But let's use the equation α2018=11α+24\alpha^{2018} = 11\alpha + 24.

So,
α2018=11α+2411×(1+342007)+24=11+11×342007+24=35+3742007 \alpha^{2018} = 11\alpha + 24 \approx 11 \times (1 + \frac{34}{2007}) + 24 = 11 + 11 \times \frac{34}{2007} + 24 = 35 + \frac{374}{2007}
Now,
37420070.186 \frac{374}{2007} \approx 0.186
So,
α201835.186 \alpha^{2018} \approx 35.186
Therefore,
α2018=35 \left\lfloor \alpha^{2018} \right\rfloor = 35

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.