# Conversion of Irrational roots for cubic functions and higher

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1. Jul 15, 2011

### WhiteTrout

I have been seeing a few during in my practice questions which leaves me worrying.
If it is a quadratic function, the irrational numbers can be easily obtained using the equation.

However, I got a question today which eventually took this form:

28D3+36D2-41D2+4 = 0
(I reevaluated everything a few times and I think it is correct)

And it reminded me of various other instances where this also happened.

I punched the numbers into my calculator (still using a Casio fx-570MS) but all hell broke loose when all three roots are irrational.

So, I would like to ask if there is any way to convert an irrational root into a number in terms of its squareroot? Lecturer assured there shouldn't be many of this kind of questions around, but at least it is good to be informed.

The easier way is to get a more up to date calculator. I recall there was one or two capable of converting to squareroots.

2. Jul 15, 2011

### ehild

Is not it

28D3+36D2-41D1+4 = 0?

Before deciding that all roots are irrational (it is impossible for a third-order equation) try to find the extrema and the limits both at -∞ and ∞. If a continuous function is negative somewhere and positive somewhere else, it has to be zero in a point in between.

ehild

3. Jul 16, 2011

### HallsofIvy

Staff Emeritus
You can use the "rational root theorem". Any rational root of that equation must be of the form 1/p where p is an integer factor of 28. Those are, of course, $\pm 1, \pm 1/2, \pm 1/4, \pm 1/7, \pm 1/14, \pm 1/28$.

4. Jul 16, 2011

### LCKurtz

Presumably you mean not all the roots are complex.

5. Jul 16, 2011

### HallsofIvy

Staff Emeritus
I wondered that myself but I think he meant that, for an equation like this, with integer coefficients, all three roots cannot be real, irrational numbers (the irrational numbers being a subset of the rational numbers). If $\alpha$, $\beta$, and $\gamma$, are roots, then the equation is of the form $a(x- \alpha)(x- \beta)(x- \gamma)= ax^3+ a(\alpha+\beta+ \gamma)x^2+ a(\alpha\beta+ \alpha\gamma+ \beta\gamma)x+ a\alpha\beta\gamma= 0$. If all three of $\alpha$, $\beta$, and $\gamma$ are irrational, the coefficients cannot all be integer.

Last edited: Jul 17, 2011
6. Jul 16, 2011

### LCKurtz

Yes, I agree that may very well be what he meant.

7. Jul 16, 2011

### ehild

I meant complex instead of irrational, silly me.
ehild

8. Jul 16, 2011

### Char. Limit

The funny thing is, Halls of Ivy, it seems to me, proved your original statement correct.

9. Jul 16, 2011

### ehild

All roots looks irrational solved by Mapple {x = -2.038433315}, {x = .1088438114}, {x = .6438752182}. From the possible rational roots p/q (p, q are relative primes, p is integer divisor of 4 and q is that of 28) none is close to these roots.

ehild

10. Jul 16, 2011

### Char. Limit

Ahhh... then what's wrong with HallsOfIvy's statement? Because if there's no rational roots, his statement must be wrong.

11. Jul 17, 2011

### ehild

He did not prove it, just said that my wrong statement might have been be true which was not.

ehild

12. Jul 17, 2011

### HallsofIvy

Staff Emeritus
What I showed was that if all three roots of a cubic equation are real and irrational then at least one coefficient must be irrrational also- so a cubic equation like this, with integer coefficients, canot have all three roots irrational.

13. Jul 17, 2011

### I like Serena

@HallsofIvy: Interesting. But I don't quite get it.
You have 3 expressions for a0, a1, and a2, with 3 possibly irrational unknown roots.
Why would there not be a solution for that with integer coefficients?

Especially since the problem at hand appears to have 3 irrational roots.

Actually, I thought you proved that all roots had to be irrational.
Because if any root were rational, it would have to be one of the fractions you mentioned, but none of those satisfy the equation.
At least, that is what I got from your previous explanation and from the wiki page: http://en.wikipedia.org/wiki/Rational_root_theorem.

@OP: Sorry to be diverging from your problem.

14. Jul 17, 2011

### gb7nash

If the rational root test fails, this only means that there are no rational solutions. You can still have irrational or complex-valued solutions.

15. Jul 18, 2011

### WhiteTrout

No worries, the resulting discussion is interesting to read actually; in a way it answers the question pretty well.

Otherwise I was just speculating if the answer be in terms of
[[a/$\sqrt[3]{n}$];[b/$\sqrt[3]{n}$];[c/$\sqrt[3]{n}$]
Or anything similar.

Uhm. But referring to previous posts... Probably not that simple.
Didn't think that one out too well did I.

Last edited: Jul 18, 2011
16. Jul 18, 2011

### uart

Hi WhiteTrout, you still haven't answered if the equation you're considering is,

28D3-5D2+4 = 0?

or

28D3+36D2-41D+4 = 0?

17. Jul 18, 2011

### HallsofIvy

Staff Emeritus
If $\alpha$, $\beta$, and $\gamma$ are roots of a cubic, it must be of the form
$$a(x- \alpha)(x- \beta)(x- \gamma)= ax^3- a(\alpha+\beta+ \gamma)x+ (\alpha\beta+ a\alpha\gamma+ \beta\gamma)x+ a\alpha\beta\gamma= 0$$

Now, is it possible for $\alpha$, $\beta$, and $\gamma$ to be irrational if a, $a(\alpha+ \beta+ \gamma)$, $a(\alpha\beta+ \alpha\gamma+ \beta\gamma$, and $a\alpha\beta\gamma$ are all rational?

18. Jul 18, 2011

### WhiteTrout

@uart: Quite sure it is the one in the OP.

@HallsofIvy:
Thank you for reminding me about that post too.
From referring to the thread itself, it seems unlikely for α, β, and γ to hold irrational numbers when the coefficient themselves are rational.

But in that case, the resultant answers from various calculators still leaves me baffled.
It almost seem like the formulation of the question was fired blindly, or perhaps some sort of typo; Which could probably say quite a lot about this.

If it helps the original question looks like this
28y'''' - 20y''' -113y'' +82y' +4y' -8y =0

For some reason, 82y' and 4y' are separated in the question. I unwittingly combined them because they are of the same term.
Converting it into a quartic function in terms of D, the first solution is D= 2
Synthetic division reduces it into

28D3+36D2-41D1+4 =0

Otherwise, (82+4)y' could seem like the problem, then maybe it did not correctly belong there?

19. Jul 18, 2011

### uart

@uart: Quite sure it is the one in the OP.

If the original problem was $$28y^{(5)} - 20y^{(4)} -113y^{(3)} +82y^{(2)} +4y^{(1)} -8y =0$$ the roots are all rational, so I think the original problem suffers from typos.