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## Main Question or Discussion Point

Prove that sqrt2 + sqrt6 is irrational.

Where do I start with this?

Where do I start with this?

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- Thread starter StephenPrivitera
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- #1

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Prove that sqrt2 + sqrt6 is irrational.

Where do I start with this?

Where do I start with this?

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- #2

Hurkyl

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Or alternatively, consider the integer polynomial that has that number as one of its roots.

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Suppose it is rational. Then,

sqrt2 +sqrt6 = p/q

With some other rational-irrational proofs, I squared both sides. So,

2 +2sqrt12 +6 =p

4(sqrt3+4)=

(2q)

Now what?

As for the second part, I'm not sure I know what you mean by "integer" polynomial. Do you mean a polynomial whose domain is the integers?

(x - (sqrt2 + sqrt6))(x - r)=0

x^2 - x(sqrt2 +sqrt6) -xr +r(sqrt2 + sqrt6) = 0

(sqrt2 + sqrt6)(x -r) = x(x-r)

Let's hope x does not = r.

Then sqrt2 + sqrt6 = x

....

This of course makes perfect sense, since x doesn't equal r, it must equal it's other root. What pointless work! Certainly, I don't understand what you mean.

- #4

Hurkyl

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Solve for sqrt(3)!(2q)^{2}(sqrt3+4)=p^{2}

Now what?

I meant one whose coefficients are integers...As for the second part, I'm not sure I know what you mean by "integer" polynomial. Do you mean a polynomial whose domain is the integers?

x = sqrt(2) + sqrt(6)

x^2 = 8 + 4 sqrt(3)

x^2 - 8 = 4 sqrt(3)

x^4 - 16 x^2 + 64 = 48

x^4 - 16 x^2 + 16 = 0

And depending on how much you remember about solving polynomials, the result is trivial from here.

- #5

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You can square both sides and you get something quite similar to your second method.

3=(p/2q)^4 - 8(p/2q)^2 +16

or 0=(p/2q)^4 - 8(p/2q)^2 +13

I could solve this, but the result is obvious. I'd just be undoing the square I just applied to both sides.

(p/2q)^2=4+sqrt3

So that's not right...

Clearly, you want me to solve the polynomial you wrote. But then I get,

x

So, x=2sqrt(2+sqrt3)

What nonsense is this!

Am I getting anywhere near a proof of anything besides my ignorance?

Please help.

- #6

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If a is rational and b is irrational is a+b necessarily irrational? I say yes, but suppose not. Then,

a+b=p/q for some p and q in J

and a=c/d for some c,d in J

then b=p/q-c/d=(pd-cq)/(pd)

Now if we can believe that the integers are closed under * and +, then we have derived a contradiction since pd-cq will be an integer and pd will be an integer. But how can I show they in fact are closed under * and +?

What if a and b are both irrational?

Not necessarily because

pi+(-pi)=0

Does that suffice?

How can I know that the additive inverse of an irrational is also irrational?

- #7

Hurkyl

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Nah, this is one of those things that you usually have to see a few times before the method sinks in.Ok, wow, you're going to think I'm really incompetent.

When you solved for sqrt(3), you got:

sqrt(3) = (p/(2q))^2 - 4

We assumed that sqrt(2) + sqrt(6) is rational, and from that assumption we've proven that there is some integers p and q so the above equation holds... the left hand side of this equation is an irrational number, but what about the right hand side?

As for the integer polynomial, there is a theorem that if p/q is a rational root of an integer polynomial in lowest terms, then p divides the constant term and q divides the leading coefficient. For example, if the equation

2x^3 - 3x + 6 = 0

has any rational roots, they must be among these possibilities:

1/1, -1/1, 2/1, -2/1, 3/1, -3/1, 6/1, -6/1, 1/2, -1/2, 3/2, -3/2

so you just have to exhaust all the possibilities to prove that this polynomial has no rational roots.

So try this theorem with the polynomial

x^4 - 16 x^2 + 16 = 0

for which we know sqrt(2) + sqrt(6) is a root.

- #8

Hurkyl

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But how about this one:

prove or disprove: If n is a natural number, then

41 + 2 + 4 + 6 + ... + 2n is prime.

My main problem is understanding what the pattern is. To me this is like asking me to prove that any sum of numbers is prime. Can you see a pattern? Maybe the first term is supposed to be one (not 41)? Then the pattern would be 1+2+4+6+8+10+12+...+2n, right?

- #10

Hurkyl

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If I had to guess, I think they mean:

41 + Σ2n

41 + Σ2n

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