True or false :
The sum of two irrational numbers is always irrational.
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Are there any non-trivial counter-examples?
Class!!!!!!
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.
Subtracting an irrational number from itself is not summating two irrational numbers.
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Technically the answer given by subtracting a number from itself DOES give a rational number but the question states ADDING!
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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)
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.
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.
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.
m , n ∈ Q ??
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
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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
The second example is incorrect because 0 . 3 3 3 ⋯ = 3 1 and 0 . 6 6 6 ⋯ = 3 2 and hence both of them are rational.
An example of irrational + irrational = rational based on decimal expansion is
0 . 1 0 1 1 0 0 1 1 1 0 0 0 1 1 1 1 0 0 0 0 . . . + 0 . 0 1 0 0 1 1 0 0 0 1 1 1 0 0 0 0 1 1 1 1 . . . = 0 . 1 1 1 . . . = 9 1
best and easiest
Didn't see this coming. Good job! :D
exactly. my thought was pi + (-pi)...
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.
The sum of any two irrational numbers of the form a 1 + b c and a 2 − b c where a 1 , a 2 , b , c ∈ 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 And as both a 1 and a 2 are rational, therefore by the closure property of rational numbers, their sum is also rational.
Must all counter examples be of this form?
No, it could be a+b(pi) etc. Many potential forms, maybe an infnite number.
Awesome proof!
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?
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They could and we would. But that doesn’t bear on the truth or falsity of the original statement.
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Can the irrationals be different? For example e and Pi?
2 is irrational and − 2 is irrational too.
But 2 − 2 = 0 , which is rational.
Simple counter example :)
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... = 9 1
Therefore, the premise is false.
Thank you for not taking the trivial solution sqr(a) and -sqr(a) like many others!
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.
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.
Take any irrational x , then − x is also irrational.
Their sum is 0 ∈ 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.
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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.
Well, ( 3 + Pi ) + ( 3 - Pi ) = 6
False. The sum of two irrational numbers is NOT necessarily an irrational number. E.g. e + (1 - e) = 1
For any irrational number k , − k is irrational, and their sum is obviously 0 .
(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
If you add an irrational number to its negative counterpart, the result is zero.
π+(4-π)=4 or, for that matter... π+(-π)=0 These are both two irrational numbers that add up to whole numbers
Some irrational numbers can be added to equal 0, which is rational.
For example, π + − ( π ) = 0
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 which is irrational and positive, and 3 − e which is also irrational and positive. And 3 + e + 3 − e = 6 which is not irrational.
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Right. I guess I was specifically pointing out that you can do this easily with subtraction.
How about (Sin A)^{2} + (Cos A)^{2} = 1
For every irrationnal number x ∈ ℜ you can always do x + ( − x ) = 0 .
Answer is false.
Counter Example:
Let's take the irrational number π. π = 3 . 1 4 1 5 9 2 6 5 3 5 8 . . .
Any whole number minus π is also irrational. 1 0 − π = 6 . 8 5 8 4 0 7 3 4 6 4 1 . . .
π + ( 1 0 − π ) = 1 0
Since all whole numbers are rational, the statement is false.
In general the sum of the irrational numbers a + b and c − b where a , c ∈ Q and b , d ∈ Q + and b and d are square-free.
π is irrational and ( 1 − π ) is irrational but π + ( 1 − π ) = 1 which is obviously rational
A rational number minus an irrational number will give another irrational number. You can prove this by contradiction. If q p - x = b a , where x is an irrational number, and p , q , a , b are whole numbers, you can see reach a contradiction because x equals to ( q p - b a ), which is a rational number.
An example of sum of two irrational numbers is π + (1 - π ), 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.
Yeah... But 1/3 and 2/3 are both RATIONAL - because they are both written into the form p/q, and NOT IRRATIONAL.
Let x be any irrational number. Then − x is irrational too. And x − x = 0 , a rational number.
I thought of (4 - pi) being irrational. If you add pi, you get 4.
False. Counterexample: Pi is irrational. So is (1 - Pi). The sum of these to irrational numbers is 1, a rational number.
Square root of two - the square root of two = what?
And "what?" is clearly rational, and whole, though not natural
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?
I think that was a typing error...
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Answer is false
CounterExample:
If m = 1 + 3 and n = 2 − 3 , then m + n = 1 + 3 + ( 2 − 3 ) = 3
The number 3 can be written in the form q p as 1 3
Thus if 3 is rational then m + n is rational which contradicts the claim stated above. Therefore the claim is false.