Fraction Tower

Algebra Level 3

Let us define a fraction tower be fraction like this: 8 4 2 1 \dfrac{\;\;\;8\;\;\;}{\dfrac{\;4\;}{\dfrac{2}{1}}}

That is, when we look from top to bottom, the fraction line is shorter and shorter and the numbers are decreasing power of 2.

What is the value of fraction tower with 2 2016 2^{2016} at the top?
Let the answer be 2 A 2^A , enter A A as your answer.


The answer is 1008.

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3 solutions

Chew-Seong Cheong
Mar 31, 2016

We note that the 4 4 -level fraction tower can be written as 8 4 2 1 = 8 4 2 1 \dfrac{\;\;\;8\;\;\;}{\dfrac{\;4\;}{\dfrac{2}{1}}} = \dfrac{8}{4}\cdot{}\dfrac{2}{1} .

Therefore, the 2017 2017 -level fraction tower is as follows:

Q = 2 2016 2 2015 2 2014 2 2013 2 2012 2 2011 . . . 2 6 2 5 2 4 2 3 2 2 2 1 = n = 1 1008 2 2 n 2 2 n 1 = n = 1 1008 2 = 2 1008 \begin{aligned} Q & = \frac{2^{2016}}{2^{2015}} \cdot{} \frac{2^{2014}}{2^{2013}} \cdot{} \frac{2^{2012}}{2^{2011}} \cdot{} ... \frac{2^6}{2^5} \cdot{} \frac{2^4}{2^3} \cdot{} \frac{2^2}{2^1} \\ & = \prod_{n=1}^{1008} \frac{2^{2n}}{2^{2n-1}} \\ & = \prod_{n=1}^{1008} 2 = 2^{1008} \end{aligned}

A = 1008 \Rightarrow A = \boxed{1008}

Cool pairing-up solution!
Actually when I solve it myself, I look for a pattern:
1 1 -level = 1 =1
2 2 -level = 2 =2
3 3 -level = 2 =2
4 4 -level = 4 =4
...
2017 2017 -level = 2 2017 2 = 2 1008 \displaystyle=2^{\lfloor \frac{2017}2 \rfloor}=2^{1008}


展豪 張 - 5 years, 2 months ago

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I got that too. But then I noticed it is easier to understanding by flipping the fractions.

Chew-Seong Cheong - 5 years, 2 months ago

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Yes! And yours is a more formal proof!

展豪 張 - 5 years, 2 months ago

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@展豪 張 Or rather, an actual proof.

Jake Lai - 5 years, 2 months ago

The first step is already invalid. 8/4/2/1 is equal to, according to the way you did, (8/4) * (1/2) not (8/4)*(2/1)

Timothy Vu - 5 years ago

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Could you explain a bit more why it should be 8 4 × 1 2 \dfrac 84\times\dfrac 12 ?

展豪 張 - 5 years ago

It's definitely 8 4 × 2 1 \frac { 8 }{ 4 } \times \frac { 2 }{ 1 } .

An easy way to see it is

8 4 2 1 = 8 ÷ 4 2 1 = 8 × 2 1 4 = 8 4 × 2 1 \frac { 8 }{ \frac { 4 }{ \frac { 2 }{ 1 } } } =8\div \frac { 4 }{ \frac { 2 }{ 1 } } =8\times \frac { \frac { 2 }{ 1 } }{ 4 } =\frac { 8 }{ 4 } \times \frac { 2 }{ 1 } .

Kenneth Choo - 5 years ago

Okay guys, the way I solved it was probably not the most efficient way, but it is pretty intuitive.
What I like the most is that it is easy to see the pattern here.

So the general answer of this tower fraction for 2 n 2^{n} , were n N n \in \mathbb{N} is 2 n 2 \displaystyle 2^{\lceil \frac{n}{2} \rceil} .

Obs: Sorry for my handwriting, when I wrote this I was trying to make sense to myself, just later that I thought of posting this solution (regardless the fact that there were already other excellent solutions).

Obs2: n \lceil n \rceil denotes the ceiling function.

Moderator note:

Congrats on guessing the general formula. How can we prove it?

Nice graph! Plotting a graph can help us find a pattern.

展豪 張 - 5 years ago

Congrats on guessing the general formula. How can we prove it?

Calvin Lin Staff - 4 years, 12 months ago

Let f ( x ) f(x) the function defined as the fraction tower with x x at the top. f ( a b ) = a b 2 f(a^{b})=a^{\frac{b}{2}} if b b is an even number, so f ( 2 2016 ) = 2 1008 f(2^{2016})=2^{1008}

Can you prove this?

Jake Lai - 5 years, 2 months ago

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If you start to divide from the first terms you will find that a 2 a = a \frac{a^2}{a}=a So the next division will be a 3 a = a 2 \frac{a^3}{a}=a^2 a 4 a 2 = a 2 \frac{a^4}{a^2}=a^2 a 5 a 2 = a 3 \frac{a^5}{a^2}=a^3 a 6 a 3 = a 3 \frac{a^6}{a^3}=a^3 Etc, more than a proof for that "property" is a pattern

Hjalmar Orellana Soto - 5 years, 2 months ago

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