Complicated Expression

Algebra Level 4

Let x x be a solution to the equation x 2 + x + 2014 = 0 x^2+x+2014=0 .

Find the value of lim n i = 1 2 n x i 201 4 i 1 2 + 1 n . \lim\limits_{n\to \infty} \sqrt[\Large n]{\prod_{i=1}^{2n}\left|\dfrac{x^i}{2014^{\frac{i-1}{2}}}+1\right|}.


The answer is 2013.

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1 solution

Daniel Liu
Aug 6, 2014

First, note that both solutions of x 2 + x + 2014 = 0 x^2+x+2014=0 have a magnitude of 2014 \sqrt{2014} and a irrational argument. This means that as n n\to \infty , the values of x i 201 4 i 1 2 + 1 \dfrac{x^i}{2014^{\frac{i-1}{2}}}+1 where 1 i n 1\le i\le n become the set of points with distance 2014 \sqrt{2014} away from 1 1 on the complex plane (because x i 201 4 i 1 2 = 2014 \left|\dfrac{x^i}{2014^{\frac{i-1}{2}}}\right|=\sqrt{2014}

Thus, x i 201 4 i 1 2 + 1 \left|\dfrac{x^i}{2014^{\frac{i-1}{2}}}+1\right| are the distances between the circle of radius 2014 \sqrt{2014} and center 1 1 and the origin.

Also, i = 1 2 n stuff n = i = 1 n stuff n 2 = i = 1 n stuff n 2 \displaystyle\sqrt[\Large n]{\prod_{i=1}^{2n}\text{stuff}}=\sqrt[\Large \frac{n}{2}]{\prod_{i=1}^{n}\text{stuff}}=\sqrt[\Large n]{\prod_{i=1}^{n}\text{stuff}}^2

Thus we just need to find the square of the geometric mean of all the distances between the origin and the circle.

Now given any point P P on the circle, draw a line through that point and 1 1 . The line will cross the circle at another point Q Q . If we let 1 1 be point X X , then by Power of a Point, ( P X ) ( Q X ) = ( 2014 + 1 ) ( 2014 1 ) = 2013 (PX)(QX)=(\sqrt{2014}+1)(\sqrt{2014}-1)=2013 .

However, we can apply this strategy for any point P P on the circle, and since the geometric mean of any two opposite distances on the circle (the P X PX and the Q X QX ) is always ( P X ) ( Q X ) = 2013 \sqrt{(PX)(QX)}=\sqrt{2013} , then the geometric mean of the entire thing is also 2013 \sqrt{2013} .

However, we wanted the square of the geometric mean, so our answer is 2013 2 = 2013 \sqrt{2013}^2=\boxed{2013} .


I have a feeling in my gut that this answer is wrong and we need calculus.

Yes, I also think that 2013 2013 is wrong. I got 2014 2014 as follows:

Note that x = 1 + 3 i 895 2 = 2014 c i s ( π arctan ( 3 895 ) ) x=\dfrac{-1+3i\sqrt{895}}{2}=\sqrt{2014} cis (\pi - \arctan(3 \sqrt{895}))

Now, let's change i i for k k to avoid confusions. I got this after some algebra:

x k 201 4 k 1 2 + 1 = 2015 + 2 2014 cos ( k ( π arctan ( 3 895 ) ) ) \left|\dfrac{x^k}{2014^{\frac{k-1}{2}}}+1\right|=\sqrt{2015+2\sqrt{2014} \cos(k(\pi-\arctan(3\sqrt{895})))}

So, we are looking for:

lim n k = 1 2 n 2015 + 2 2014 cos ( k ( π arctan ( 3 895 ) ) ) 2 n \lim_{n \to \infty} \prod_{k=1}^{2n} \sqrt[2n]{2015+2\sqrt{2014} \cos(k(\pi-\arctan(3\sqrt{895})))}

Honestly I don't know how to solve that limit, but after some iterations I noticed that it converges to 2014 2014 . Any thoughts?

Alan Enrique Ontiveros Salazar - 6 years, 10 months ago

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I honestly have no idea.

Daniel Liu - 6 years, 10 months ago

I think Power of a Point cannot be applied to the center of the circle but could be applied to the origin and give the right answer 2013

Samer Atasi - 5 years, 2 months ago

....the answer is 2013.

The only problem in the solution is that point X X should be the origin, not 1. 1.

Boi (보이) - 3 years, 10 months ago

As i varies form 1 to 2n and limit n tends to infinity that is n belongs to natural number so their are discontinuous point on circle s I am not clear about the condition that X,P, Q are collinear if n belongs to R then answer will be 2013 but if n belongs to N then limiting value tens to 2014.

Vagish Jha - 6 years, 10 months ago

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By definition, n n is a natural number.

Alan Enrique Ontiveros Salazar - 6 years, 10 months ago

Is that so? That's pretty interesting. So is the answer 2014 2014 after all?

Daniel Liu - 6 years, 10 months ago

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