Irrational + Irrational = ?

True or false :

The sum of two irrational numbers is always irrational.

True False

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28 solutions

Joe Lee
Mar 3, 2016

Answer is false

CounterExample:

If m = 1 + 3 and n = 2 3 m = 1+\sqrt{3} \hspace{3mm} \text{and} \hspace{3mm} n = 2 - \sqrt{3} , then m + n = 1 + 3 + ( 2 3 ) = 3 m + n = 1 + \sqrt{3} + \left(2 - \sqrt{3}\right) = 3

The number 3 3 can be written in the form p q \frac{p}{q} as 3 1 \frac{3}{1}

Thus if 3 3 is rational then m + n m+n is rational which contradicts the claim stated above. Therefore the claim is false.

Are there any non-trivial counter-examples?

Cale Nearing - 5 years, 3 months ago

Class!!!!!!

Kartik Tyagi - 5 years, 3 months ago

technically there is only one irrational number in your proof, you just used it twice and canceled it out, so you have only really shown that m + n = 1 + 2 = 3.

Ben Robinson - 2 years, 5 months ago

Subtracting an irrational number from itself is not summating two irrational numbers.

A Former Brilliant Member - 2 years, 5 months ago

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Technically the answer given by subtracting a number from itself DOES give a rational number but the question states ADDING!

harry warner - 2 years, 5 months ago

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(2 minus root 3) is a positive irrational number — and it’s being added to (root 3). So this demonstrates that it is possible to generate a rational number from the sum of two irrationals.

(It actually doesn’t matter if they’re positive! Irrational and rational numbers can be negative, so you could actually sum any combination of positive and negative irrationals and call it addition)

Grant Petersen - 2 years, 2 months ago

Mathew. Square root of 3 is a real number but cannot be expressed as a ratio, i.e. it is irrational. It is not infinite. Its value is somewhere between 1.70 and 1.75. Hence it can be added, subtracted, multiplied or divided.

Munir Gilani - 2 years, 4 months ago

It states summing two irrational number, it does not state one of them is negative or both have the same sign. If we assume both numbers are positive, their sum will not always be rational. For example, sqrt(2)+pi is irrational. I think the problem is stated improperly and (always) should be replaced or removed.

Abdul Aziz Abdul Majid - 2 years, 2 months ago

Let n be a irrational number. n is irrational, and -n should be irrational. n + -n = n - n = 0 Since 0 is not irrational, the statement is false.

Tony Tu - 6 months ago

m , n Q m,n \in \mathbb Q ??

Rishik Jain - 5 years, 3 months ago

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Sorry I fixed it!

Joe Lee - 5 years, 3 months ago

I disagree with this. We do not know the exact value to the square root of 3. We can calculate this to be infinite. By definition, infinity has no value. We cannot cancel out infinite values by other infinite values

Mathew Korbin - 5 years, 3 months ago

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We can calculate it to have infinite length, not infinite quantity. Every real number can be written with an infinite decimal expansion (e.g. 1=.999...). So, by your argument, the statement x=x is false for all real numbers

Joseph Skelton - 5 years, 3 months ago
Zeeshan Ali
Mar 4, 2016

2 + ( 2 ) = 0 \sqrt{2}+(-\sqrt{2})=\boxed{0}

The second example is incorrect because 0.333 = 1 3 0.333\dots=\frac13 and 0.666 = 2 3 0.666\dots=\frac23 and hence both of them are rational.

An example of irrational + irrational = rational based on decimal expansion is

0.10110011100011110000... + 0.01001100011100001111... = 0.111... = 1 9 0.10110011100011110000... + 0.01001100011100001111... = 0.111... = \frac19

Note: both numbers on the left are irrational because their decimals are not repeating.

Arulx Z - 5 years, 3 months ago

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rIGHT! All recurring numbers are rational..!

Zeeshan Ali - 5 years, 3 months ago

best and easiest

Yashvardhan Baid - 5 years, 3 months ago

Didn't see this coming. Good job! :D

Cha Wong - 3 years, 5 months ago

exactly. my thought was pi + (-pi)...

Carlos Roldan - 3 years, 3 months ago

I used 0.10110111011110... and 0.01001000100001... but the idea is the same. Add these two irrational numbers to get 0.111111111111111... = 1/9, which of course is rational.

Tim Popely - 2 years, 5 months ago

The sum of any two irrational numbers of the form a 1 + b c a_1+b\sqrt{c} and a 2 b c a_2-b\sqrt{c} where a 1 , a 2 , b , c Q a_1,a_2,b,c\in\mathbb Q is rational. We can easily verify this as: ( a 1 + b c ) + ( a 2 b c ) = ( a 1 + a 2 ) + ( b c b c ) = a 1 + a 2 (a_1+b\sqrt{c})+(a_2-b\sqrt{c})=(a_1+a_2)+(b\sqrt{c}-b\sqrt{c})=a_1+a_2 And as both a 1 a_1 and a 2 a_2 are rational, therefore by the closure property of rational numbers, their sum is also rational.

Moderator note:

Must all counter examples be of this form?

No, it could be a+b(pi) etc. Many potential forms, maybe an infnite number.

Anu Sood - 4 years, 5 months ago

Awesome proof!

Joe Lee - 5 years, 3 months ago

why can't the examples be a1+b1(sqrt c) and a2+b2(sqrt c)? couldn't we get an irrational number if we add these?

Pavan Konathala - 3 years, 8 months ago

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They could and we would. But that doesn’t bear on the truth or falsity of the original statement.

/@

Ant Allan - 3 years, 1 month ago

Can the irrationals be different? For example e and Pi?

Rafael Bernal - 3 years, 2 months ago
Arulx Z
Mar 4, 2016

2 \sqrt{2} is irrational and 2 -\sqrt{2} is irrational too.

But 2 2 = 0 \sqrt{2}-\sqrt{2} = 0 , which is rational.

Moderator note:

Simple counter example :)

Gus O'Donnell
Aug 20, 2017

The decimal representation of an irrational number extends to infinity without repeating. Consider the two following irrational numbers:

x = 0.101001000100001000001000000...

y = 0.010110111011110111110111111...

x + y = 0.11111111... = 1 9 \frac{1}{9}

Therefore, the premise is false.

Thank you for not taking the trivial solution sqr(a) and -sqr(a) like many others!

Sylvain Robitaille - 3 years, 2 months ago

A much better effort, but I'm still concerned about the reuse of the single irrational:

x = 0.101001000100001000001000000...

y = 1/9 - x

x + y = x + 1/9 - x = 1/9

y is only irrational because it contains x. Even looking at the rule you used, y is just x's rule negated.

Ben Robinson - 2 years, 5 months ago

The counter to the claim that immediately popped into my mind was this: Take a whole number and subtract any irrational number (A) from it. The result will be irrational (B). A+B = a whole number, thus proving it is possible for two irrational numbers to add to a rational number.

Cool, best effort yet as this actually has two different irrational numbers that do not (necessarily) have any resemblance to one another. We can generalize further and say that you can take any rational number r, subtract any irrational number i from it and get another irrational number j, such that i + j = r.

Note: they could still have resemblance in some cases such as r=0, where i = -j, but this becomes the exception, not the rule.

Ben Robinson - 2 years, 5 months ago
Kevin Amaral
Mar 9, 2016

Take any irrational x x , then x -x is also irrational.

Their sum is 0 Q 0 \in \mathbb Q , a rational number.

Therefore, the claim is false.

i dont think 2^.5 - 2^.5 = 0 is the jist of the question. My interpretation is more along the lines of the sum of 2 irrational numbers (not being the opposites of each other hence algebraically eliminated) can not be a rational number, eg, pi + e or something equevilent.

Nicholas Legge - 2 years, 10 months ago

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it's spelled 'gist', and i think he has understood the gist, just found a simple counter example that may not come to mind immediately for some.

jay poole - 2 years, 5 months ago

Well, ( 3 + Pi ) + ( 3 - Pi ) = 6

Gary Dubler - 2 years, 9 months ago
Will Clawson
Sep 3, 2017

False. The sum of two irrational numbers is NOT necessarily an irrational number. E.g. e + (1 - e) = 1

Kal Hourani
Mar 9, 2016

For any irrational number k k , k -k is irrational, and their sum is obviously 0 0 .

John Dinglong
Jul 8, 2018

(Phi^2)+(-phi)=1 as does (phi) +(phi^(-1)), both are counter examples, if you're looking for a solution that is two positive numbers you can simply add any rational number to both addends

E Db
Dec 8, 2017

If you add an irrational number to its negative counterpart, the result is zero.

Patrick Beranek
Nov 26, 2017

π+(4-π)=4 or, for that matter... π+(-π)=0 These are both two irrational numbers that add up to whole numbers

Keshav Ramesh
Jun 4, 2017

Some irrational numbers can be added to equal 0, which is rational.

For example, π + ( π ) = 0 \pi+-(\pi)=0

Oli Hohman
Mar 9, 2016

This is too easy if you don't exclude negative negative numbers. e is irrational. -e+e = 0, which is not irrational. You can do the same thing for any irrational number.

It doesn't mind. It's as easy even excluding negative numbers.

You can take 3 + e 3+e which is irrational and positive, and 3 e 3-e which is also irrational and positive. And 3 + e + 3 e = 6 3+e+3-e=6 which is not irrational.

David Molano - 5 years, 3 months ago

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Right. I guess I was specifically pointing out that you can do this easily with subtraction.

Oli Hohman - 5 years, 3 months ago
Verne Cabading
Mar 9, 2016
  • Let x and y be irrational numbers √2 and 1 - √2 respectively.
  • √2 + (1 - √2) = 1.
  • x + y = 1 which is a rational number contradicting the statement. Therefore, the answer is false.
Bill Kaufman
Jan 19, 2019

How about (Sin A)^{2} + (Cos A)^{2} = 1

Julien Aublin
Jun 18, 2018

For every irrationnal number x x\in \Re you can always do x + ( x ) = 0 x + (-x) = 0 .

Answer is false.

Counter Example:

Let's take the irrational number π. π = 3.14159265358... π=3.14159265358...

Any whole number minus π is also irrational. 10 π = 6.85840734641... 10-π=6.85840734641...

π + ( 10 π ) = 10 π+(10-π)=10

Since all whole numbers are rational, the statement is false.

In general the sum of the irrational numbers a + b a+\sqrt{b} and c b c-\sqrt{b} where a , c Q a,c \in Q and b , d Q + b,d \in Q^+ and b and d are square-free.

π \pi is irrational and ( 1 π ) (1-\pi) is irrational but π + ( 1 π ) = 1 \pi+(1-\pi)=1 which is obviously rational

Wai Yee Low
Aug 25, 2017

A rational number minus an irrational number will give another irrational number. You can prove this by contradiction. If p q \frac{p}{q} - x {x} = a b \frac{a}{b} , where x {x} is an irrational number, and p {p} , q {q} , a {a} , b {b} are whole numbers, you can see reach a contradiction because x {x} equals to ( p q \frac{p}{q} - a b \frac{a}{b} ), which is a rational number.

An example of sum of two irrational numbers is π {\pi} + (1 - π {\pi} ), which equals to 1. In fact, the sum of an irrational number with one minus the same irrational number always give a rational number. So the claim is false.

Lass Kobani
Jul 21, 2017

1/3 + 2/3 = 1

Yeah... But 1/3 and 2/3 are both RATIONAL - because they are both written into the form p/q, and NOT IRRATIONAL.

J.J. The Great - 3 years, 2 months ago
David Molano
Mar 11, 2016

Let x x be any irrational number. Then x -x is irrational too. And x x = 0 x-x=0 , a rational number.

π + ( 4 π ) = 4 \pi+(4-\pi)=4

Hans P
Nov 20, 2018

I thought of (4 - pi) being irrational. If you add pi, you get 4.

Sam Shepherd
Sep 18, 2018

False. Counterexample: Pi is irrational. So is (1 - Pi). The sum of these to irrational numbers is 1, a rational number.

Robert Louviere
Mar 14, 2016

Square root of two - the square root of two = what?

And "what?" is clearly rational, and whole, though not natural

Barry Vanderhorst - 3 years ago
Poh Seng Tan
May 21, 2018
Number 1 0.689 552 517 401 407 573 575 438 761 76... Irrational
Number 2 5.310 447 482 598 592 426 424 561 238 23... rational
Sum of numbers 5.999 999 999 999 999 999 999 999 999 99...=6 rational

Why is number 2 rational?

Ben Robinson - 2 years, 5 months ago

I think that was a typing error...

Poh Seng Tan - 1 year, 11 months ago

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