# Thread: Rational and Irrational numbers

1. ## Rational and Irrational numbers

Now, don't think this is just an ordinary question! Here goes:

a and b are irrational numbers
a + b is rational
a x b is rational

Work out possible values for a and b where they are positive irrational numbers.

I am sure that I have gotten down all the details, any help will be much appreciated, thank you!

2. Originally Posted by Geometor
Now, don't think this is just an ordinary question! Here goes:

a and b are irrational numbers
a + b is rational
a x b is rational

Work out possible values for a and b where they are positive irrational numbers.

I am sure that I have gotten down all the details, any help will be much appreciated, thank you!

a = (1 + sqrt(2))
b = -sqrt(2)
then a and b are irrational numbers and...

a + b = (1 + sqrt(2)) + (-sqrt(2)) = 1 which is rational

a = sqrt(2)
b = 1/sqrt(2)
then a and b are both irrational and...

a*b = sqrt(2)*1/sqrt(2) = 1 which is rational

i should be using this oppurtunity to try out Latex, i don't know how to use it

EDIT: o sorry, i forgot a and b must be positive, so the first example is incorrect, i'll come up with another one

a = 10 - sqrt(5) + sqrt(2)
b = sqrt(5) - sqrt(2)

then a + b = 10 which is rational and both a and b are positive irrational numbers

3. Originally Posted by Jhevon
a = (1 + sqrt(2))
b = -sqrt(2)
then a and b are irrational numbers and...

a + b = (1 + sqrt(2)) + (-sqrt(2)) = 1 which is rational

a = sqrt(2)
b = 1/sqrt(2)
then a and b are both irrational and...

a*b = sqrt(2)*1/sqrt(2) = 1 which is rational

i should be using this oppurtunity to try out Latex, i don't know how to use it

EDIT: o sorry, i forgot a and b must be positive, so the first example is incorrect, i'll come up with another one

a = 10 - sqrt(5) + sqrt(2)
b = sqrt(5) - sqrt(2)

then a + b = 10 which is rational and both a and b are positive irrational numbers
Many thanks but I don't think that is correct since
10 - sqrt(5) + sqrt(2) times sqrt(5) - sqrt(2) is irrational :S
thanks for trying, lol this is a really complex problem ay?

is this a trick question and it might be impossible?

4. Originally Posted by Geometor
Now, don't think this is just an ordinary question! Here goes:

a and b are irrational numbers
a + b is rational
a x b is rational

Work out possible values for a and b where they are positive irrational numbers.
$\displaystyle a=1-\sqrt{2} \mbox{ and }b=1+\sqrt{2}$

5. Originally Posted by ThePerfectHacker
$\displaystyle a=1-\sqrt{2} \mbox{ and }b=1+\sqrt{2}$
erm... the question requires "positive" numbers and I believe that 1- sq. root 2 is negative?

thanks for helping anyway

6. Originally Posted by Geometor
Many thanks but I don't think that is correct since
10 - sqrt(5) + sqrt(2) times sqrt(5) - sqrt(2) is irrational :S
thanks for trying, lol this is a really complex problem ay?

is this a trick question and it might be impossible?
o, we have to come up with a pair of numbers that satisfy both condtions at the same time? i thought we could use different examples for each.

EDIT: TPH came up with an example

7. Originally Posted by Geometor
erm... the question requires "positive" numbers and I believe that 1- sq. root 2 is negative?

thanks for helping anyway
So then change it a little bit,

$\displaystyle a = 2 - \sqrt{2} \mbox{ and } b = 2 + \sqrt{2}$

What is so hard?

8. Originally Posted by Geometor
Now, don't think this is just an ordinary question! Here goes:

a and b are irrational numbers
a + b is rational
a x b is rational

Work out possible values for a and b where they are positive irrational numbers.

I am sure that I have gotten down all the details, any help will be much appreciated, thank you!
Because their sum is rational $\displaystyle a$ and $\displaystyle b$ are of the form:

$\displaystyle a = A + x$,

$\displaystyle b = B - x$

where $\displaystyle A$ and $\displaystyle B$ are rational and x is irrational.

Then:

$\displaystyle a\ b = AB + (-A+B)x - x^2 = C$

where C is rational.

Now we have demanded that A, B and C be rational, but we may as well
demand that they be integers (the derivation of rational solutions from
integer solutions is fairly elementary and left to the reader).

Anyway we have:

$\displaystyle x^2 + (A-B)x + (C- AB) = 0$

and $\displaystyle x$ is irrational. However:

$\displaystyle x=\frac{-(A+B) \pm \sqrt{(A-B)^2 - 4(C-AB)}}{2}=\frac{-(A+B) \pm \sqrt{(A+B)^2 - 4C}}{2}$

which is irrational only if $\displaystyle q= (A+B)^2 - 4C$ is not a perfect
square.

So here is our algorithm for finding solutions:

Choose an integer $\displaystyle K$, and an integer $\displaystyle C$ such that $\displaystyle K^2 - 4C$ is not a perfect square,
then choose $\displaystyle A$ and $\displaystyle B$ so that $\displaystyle A+B=K$, then:

$\displaystyle x=\frac{-(A+B) \pm \sqrt{(A+B)^2 - 4C}}{2}$

is an irrational number such that if:

$\displaystyle a=A+x$
$\displaystyle b=B-x$

then $\displaystyle a$ and $\displaystyle b$ are irrational and $\displaystyle a+b$ is an integer as is $\displaystyle a \times b$

Of course this does not guarantee that both $\displaystyle a$ and $\displaystyle b$ are positive.

RonL

9. thanks for the help! sorry, can only thank one person a day I believe?
I find this very confusing though :S

10. Originally Posted by Geometor
thanks for the help! sorry, can only thank one person a day I believe?
I find this very confusing though :S
No you can thank as many as you want.

RonL

11. Originally Posted by Geometor
thanks for the help! sorry, can only thank one person a day I believe?
how come?

12. Originally Posted by Jhevon
how come?
oh woops, nevermind it's just that I can't thank on the same post so I got mixed up