Divide divide only divide...

Algebra Level 1

True or False?

1 ÷ ( 2 ÷ ( 3 ÷ ( 4 ÷ ( ÷ ( 2015 ÷ ( 2016 ÷ 2017 ) ) ) > 1 \large 1\div\Big(2\div (3 \div (4\div ( \cdots \div (2015\div (2016\div 2017) ) \cdots \Big) > 1

True False

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16 solutions

Q = 1 ÷ ( 2 ÷ ( 3 ÷ ( 4 ÷ ÷ ( 2015 ÷ ( 2016 ÷ 2017 ) ) ) = 1 × 3 × 5 × × 2015 × 2017 2 × 4 × 6 × × 2016 = 3 2 × 5 4 × 7 6 × × 2 k + 1 2 k × × 2017 2016 > 1 where k N \begin{aligned} Q & = 1 \div (2 \div (3 \div (4 \div \cdots \div (2015 \div (2016 \div 2017))\cdots ) \\ & = \frac {1\times 3 \times 5 \times \cdots \times 2015 \times 2017}{2 \times 4 \times 6 \times \cdots \times 2016} \\ & = \frac 32 \times \frac 54 \times \frac 76 \times \cdots \times \frac {2k+1}{2k} \times \cdots \times \frac {2017}{2016} > 1 & \small \color{#3D99F6} \text{where }k \in \mathbb N \end{aligned}

Note that the nominator of each term is 1 larger than denominator. Therefore Q > 1 Q > 1 and the expression is true .

Sir, Your solutions are much clear and beautiful.Thank you for sharing.

Naren Bhandari - 3 years, 7 months ago

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You are welcome.

Chew-Seong Cheong - 3 years, 7 months ago

I don't understand the second line, sir. Why did you put the odds as dividend and the evens as divisor?

Mario Freire - 3 years, 7 months ago

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Ah! Now I get it. 2015 divided by 2016 divided by 2017 is equal to (2015 x 2017) divided by 2016. All right.

Mario Freire - 3 years, 7 months ago

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Good that you get it. 1 2 3 . . . = 1 2 × 3 . . . \dfrac 1{\frac 23 ...} = \dfrac 1{2} \times \dfrac 3{...}

Chew-Seong Cheong - 3 years, 7 months ago

Very nicely explained. Thank you!

Emma Boonzaier - 3 years, 7 months ago

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You are welcome

Chew-Seong Cheong - 3 years, 7 months ago

Very clear explanation sir !(+1)

Rishu Jaar - 3 years, 7 months ago

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Thanks for liking the solution.

Chew-Seong Cheong - 3 years, 7 months ago

It is only true if the final in sequence is odd, however they have put ...) at the end which means it is an infinite sequence. Is infinity an even or odd number, I don't know? Do you?

Rhonda Dyane - 3 years, 7 months ago

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Yes, you can try a few terms first to see this. 1 2 < 1 \dfrac 12 < 1 , 1 2 3 = 1 × 3 2 > 1 \dfrac 1{\frac 23} = \dfrac {1\times 3}2 > 1 , 1 × 3 2 × 4 < 1 \dfrac {1\times 3}{2 \times 4}< 1 .... If it is infinite, it is not defined.

Chew-Seong Cheong - 3 years, 7 months ago

The ...) at the end specifies closing all of the parentheses, not continuing the sequence of divisors.

Stephen Beck - 3 years, 7 months ago

I tried this on a smaller scale on the calculator 3/2 5/4 7/6 was bigger than 1, but (3/2 5/4 7/6)≠(1/2/3/4/5/6/7)

Jens Stegeby - 3 years, 7 months ago

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I have tried it with small scale first before I wrote the answer with an Excel spreadsheet it works. It is probably that your calculations were incorrect.

Chew-Seong Cheong - 3 years, 7 months ago

I have a huge problem with the solution the dots after 2017 indicate to me that the numbers keep on going. The solutions I find here act is if the problem stops at 2017. If one looks at an even number being the last number the statement is <1, if the last number is odd then the statement is greater than 1

Richard Davis - 3 years, 7 months ago
Zain Majumder
Nov 5, 2017

If the divisor is greater than the dividend, the quotient will be less than 1 1 .

If the divisor is less than 1 1 , the quotient will be greater than the dividend.

2016 ÷ 2017 < 1 2016\div2017 < 1

Therefore, 2015 ÷ ( 2016 ÷ 2017 ) > 2015 2015\div(2016\div2017) > 2015

Therefore, 2014 ÷ ( 2015 ÷ ( 2016 ÷ 2017 ) ) < 1 2014\div(2015\div(2016\div2017)) < 1

Therefore, 2013 ÷ ( 2014 ÷ ( 2015 ÷ ( 2016 ÷ 2017 ) ) ) > 2013 2013\div(2014\div(2015\div(2016\div2017))) > 2013

And so on.

When the dividend is odd, the quotient will be greater than 1 1 . Therefore, the inequality is t r u e . \boxed{true}.

I don't agree with the answer given: the inequality as presented goes on beyond 2017, as indicated by the dots to the right of that number, hence the question implies that the inequality is true in all cases, not just when the final number is 2017. As stated above, the inequality is true if the last number is odd and false if it is even, so it is not true in all cases.

Roger Fisken - 3 years, 7 months ago

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If the expression did go on forever, the 2017 should be replaced with ellipses. In fact, if this was the case, there would be no reason to include 2015, 2016, and 2017 in the question.

The ellipses you mention signify the closing parentheses of each division.

Zain Majumder - 3 years, 7 months ago

I agree with Zain. The ellipses to the right of 2017 signify the many closing parentheses that would have to be drawn. If your statement was true then it would be written as such: 1/(2/(3/(4/(.../(2015/(2016/(2017/(...))

Brian Honsinger - 3 years, 7 months ago
Mohammad Khaza
Nov 6, 2017

look carefully,

starting from the middle-- 2016 2017 < 1 \frac{2016}{2017}<1

and then,if we divide the gotten value by 2015 2015 .it will be greater than 1 1 . example: 2015 1 = 2015 \frac{2015}{1}=2015 but 2015 . 9 ( l e s s t h a n 1 ) = 2238.88.. \frac{2015}{.9(less than 1)}=2238.88..

so,the value will increase day by day and surely the final value will be > 1 1 .

Close, but remember that if the next number (2014) is included, this will be 2014/2238.88, which is less than 1. It will alternate between greater and less than 1, it just so happens that it ends on greater than 1.

Your solution implies that the answer will be very large, but in fact it will be much closer to 1.

Zain Majumder - 3 years, 7 months ago
Md Mehedi Hasan
Nov 5, 2017

2016 ÷ 2017 < 1 2016\div2017<1

2015 ÷ ( 2016 ÷ 2017 > 1 2015\div(2016\div2017>1

2014 ÷ ( 2015 ÷ ( 2016 ÷ 2017 ) ) < 1 2014\div(2015\div(2016\div2017))<1

2013 ÷ ( 2014 ÷ ( 2015 ÷ ( ( 2016 ÷ 2017 ) ) ) ) > 1 2013\div(2014\div(2015\div((2016\div2017))))>1

\quad \vdots \quad \vdots \quad \vdots

Here we can see that if this division we start from an even number, then whole result always < 1 < 1

Again if this division we start from an odd number, then whole result always > 1 > 1

In the question, this division starts from 1 1 which is an odd number.

So logically the whole result must be > 1 > 1

So it's t r u e \color{#20A900}{\boxed{true}}

Zoe McGregore
Nov 6, 2017

When looking at the two choices I chose one at random with a 50/50 chance.

Sub to Pewdiepie
Nov 15, 2017

You can make the equation shorter -----> 1/(2/3) ----> 3/2

Arisoda Mens
Nov 11, 2017

LOL Everyone is thinking too hard I think.

I could be wrong but I noticed:

If the highest number (e.g. 2017) = odd number then it's always > 1.

If the highest number (e.g. 2018) = even number then it's always < 1

It alternates 1÷2, 1÷.66, but as 2015-1 is even and alternations on even intervals end with the opposite position as the input, it is larger than the 1.

1/(2/(3/(4/5))))>1

Sayoon Sarwar
Nov 12, 2017

As simply I would tell my answer If 1 is divided by 2 it reduces to a number lesser than one .this goes on till 2017 so the final result would be smaller than 1 Sorry I am a newbie And this what best I can do.

Frank Kelley
Nov 10, 2017

When dividing by a fraction it is the same as the numerator and denominator flipping then multiplying... 1÷ 2÷3 = 1÷2/3 = 1×3/2 > 1. Each additional '÷' added just flips another bigger number up into the denominator.

Yanick Nj
Nov 7, 2017

So I focused on the important part of the question, in my opinion. This was 2015/(2016/2017).

I think you can do this as everything else stems from it. Turn it into algebra as you do ... n/(n+1/n+2). That equals n*(n+2/n+1) which equals (n^2+2n)/(n^2+n). This produces a top heavy fraction as long as n is greater than 0, which it is.

Now I think this is alright for a solution as the whole inequality can just be represented in the same with the same method repeated all the way across.

Steve Birch
Nov 6, 2017

if n = a/b and b>a then n <1 and if b<a then n >1. The solution therefore oscillates around 1. i.e n=(>1) / (<1) / (>1) / <1) ... Therefore, if the final element is odd the solution will be >1 and if the final element is even then the solution will be <1

MauLo Qiit
Nov 6, 2017

Odd number 2017 :)

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def problem(year):
    result=year
    while year>1:
        result=(year-1)/result
        year-=1
    print (result)

This gives the answer 35.83825372...

I just started programming this week, I almost don't know anything about programming. How do you improve this code? Is it correct and how do I make it more efficient?

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#https://brilliant.org/weekly-problems/2017-11-06/basic/?p=2

def problem(year):
   result=year
   print result
   while year>1:
       result=float((year-1)*1./result)
       year-=1
   print (result)

problem(2017)
problem(2016)

Michael Fitzgerald - 3 years, 7 months ago

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1
2
2017      35.83825372692136
2016      0.017768098030203995

Michael Fitzgerald - 3 years, 7 months ago

35.83825373 is correct - according to MS Excel calculations.

Sean Sran - 3 years, 7 months ago
crashassinTM .
Nov 6, 2017

We see a pattern here which will help the sum be solved faster. In the first case the divisor is odd ,2017 which divides 2016 where the quotient is smaller than 1 so when it divides the odd dividend the quotient is greater than 1,and this pattern continues. So when an odd dividend is there quotient is greater than 1 , and the last term is 1 which is odd and thus gives quotient >1.

Simple example of the answer. Is like dividing 1 by 2 =1/2 in which it makes it < then 1. But if you ÷ 2 by 1 which still = 2 then 2 is > then 1.

Adolph Lopez - 3 years, 7 months ago

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And that's a simple answer for a simple example

Adolph Lopez - 3 years, 7 months ago

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