It is obviously true, right?

True or False?

The sum of two irrational numbers is always an irrational number.

True False

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15 solutions

Nihar Mahajan
Nov 24, 2015

Let's take the irrational numbers a + b c a+b\sqrt{c} and its conjugate a b c a-b\sqrt{c} , where c c is squarefree and a , b a,b are rational. If we add them, then we get the answer 2 a 2a which is rational.

Unless "a" is irrational but your proof makes sense. I prefer the proof with Pi or euler's irrational numbers. It's simpler.

Michael Villanueva - 5 years, 6 months ago
Sahil Kukreja
Nov 24, 2015

Pi +(-Pi) =0 & 0is rational

When I read the question, I thought Pi + (#-Pi) = #, yours is more elegant.

Crystal Hillman - 5 years ago

I'll buy this logic analogous to euler's irrational number used instead of Pi.

Michael Villanueva - 5 years, 6 months ago
Rohit Udaiwal
Nov 24, 2015

We have to prove that irrational+irrational \ne irrational.We can just put up an example to verify.E.g.. ( 4 3 3 ) + 3 3 = 4 (4-3\sqrt{3})+3\sqrt{3}=4 which is a rational number.

it only takes one counterexample to prove a statement wrong

Joseph Ferri - 3 years ago

I agree @Joseph Ferri

Neil Lunavat - 6 months, 3 weeks ago
Sirajudheen Mp
Dec 4, 2015

consider the numbers √2 and -√2 both are irrationals but sum is 0 is rational

Kunal Sharma
Nov 28, 2015

1/2(irrational)+1/2(irrational)=2/2=1 (rational)

1/2 is rational.

Linden Marquez - 5 years, 6 months ago

1/2 is a rational number.Irrational are the numbers that cannot be written in p/q form

A Former Brilliant Member - 5 years, 6 months ago

1/2 is rational.

A Former Brilliant Member - 5 years, 6 months ago

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1/2 is rational but 1/2(INSERT ANY IRRATIONAL NUMBER) is not

Cesar Ockham - 5 years, 6 months ago

Uhhhh. 1/2 is irrational?

Jonny Pei - 5 years, 5 months ago

Read the comments on @Rahul Kamble 's 'solution'

Lâm Lê - 8 months, 4 weeks ago
Sadasiva Panicker
Nov 28, 2015

root5 + -root5 = 0: 7 + root 3 = 8 - root3 = 15 ia rational

Mike Forgette
Nov 29, 2015

This is way more complicated than it needs to be, but imagine the numbers 0.45445444544445... and 0.54554555455554...

Their sum would be 1, which is rational.

Mike Trelfa
Nov 29, 2015

Seriously? How about pi plus -pi... It equals zero... This wasn't a difficult question.

Considering that x is an irrational number between 0 and 1 and that a sum of an irrational number with a rational number is a rational number: x + (1-x) = 1, which is a rational number. Therefore, a sum of irrational numbers is not always an irrational number.

Carl Lee
Nov 29, 2015

whenever we probe something is not true we need only one counter example and the example is i+(-i)=0 which is a rational number

It seems like you are confusing irrational numbers & complex numbers.

ארד קרן - 5 years, 6 months ago
. .
Feb 13, 2021

If the sum of 2 irrational numbers is always an irrational number, then 2 + ( 2 2 ) \sqrt{2}+(2-\sqrt{2}) must be an irrational number, but it is 2 2 , which is rational, so it is false that the sum of 2 irrational numbers is always an irrational number.

Lâm Lê
Sep 19, 2020

You can add together an irrational and its additive inverse to make 0 0 which is rational.

Proof: a + ( a ) = a a = 0 a+(-a)=a-a=0

John Rebo Santos
Dec 17, 2015

The title kind of gave it away actually.

Jordan Wyatt
Dec 6, 2015

Pi + (1-Pi) = 1. Pi is irrational (and transcendental btw). (1-Pi) is irrational (again transcendental btw). 1 is not irrational (nor trancendental).

Bhupendra Jangir
Nov 28, 2015

It's not always true. But for any irrational number, we can choose an irrational number such that their sum could be a rational number.

If irrational numbers are non repeating, non terminating decimals is this practically possible, our just theoretically possible?

Chris Lesmerises - 5 years, 6 months ago

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Yes.. Let (a+√b) be an irrational number. Then there exists an another irrational number (c-√b) such that their sum (a+c) is a rational number.

Bhupendra Jangir - 5 years, 6 months ago

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