Rational and irrational numbers proof

[bobbarker]

Hey all, I'm new here so I'm a little noobish at the formatting capabilities of PF. Trying my best though! :P

Homework Statement

Let a, b, c, d \in Q, where \sqrt{b} and \sqrt{d} exist and are irrational.

If a + \sqrt{b} = c + \sqrt{d}, prove that a = c and b = d.

Homework Equations

A number is rational iff it can be expressed as m/n, where m,n \in Z and n \neq 0.

A rational number added to a rational number is a rational number.

A rational number added to an irrational number is an irrational number.

A rational number multiplied by a rational number is a rational number.

A rational number multiplied by an irrational number is an irrational number.

The Attempt at a Solution

I reordered things so that

a - c = \sqrt{d} - \sqrt{b}
Then I multiply by the conjugate of \sqrt{d} - \sqrt{b} to get
(a - c)\times(\sqrt{d}+\sqrt{b}) = d - b
By the contrapositive to a different result we proved in class, since (a-c) and (d-b) are rational, either a-c = 0 or \sqrt{d}+\sqrt{b} is rational.

Now if the first case is true the result is proved, but I'm struggling with the second half of this (proving or disproving whether \sqrt{d}+\sqrt{b} can be rational). Is this the wrong direction to be going in?

Thanks.

 

[jgens]

If \sqrt{c} + \sqrt{d} is rational, then clearly its square must also be rational. So suppose \sqrt{c} + \sqrt{d} = a where a \in \mathbb{Q} and consider \sqrt{c} = a - \sqrt{d}. Can you use the first bit of information to derive a contradiction now?

 

[bobbarker]

If \sqrt{c} + \sqrt{d} is rational, then clearly its square must also be rational. So suppose \sqrt{c} + \sqrt{d} = a where a \in \mathbb{Q} and consider \sqrt{c} = a - \sqrt{d}. Can you use the first bit of information to derive a contradiction now?

Let me run this by you... I played with it for a few minutes.

(I put the problem's variables in place of yours, and defined a new quantity e\in \mathbb{Q} = \sqrt{d}+ \sqrt{b} )

So since \sqrt{b}+\sqrt{d} is rational, (\sqrt{b}+\sqrt{d})^{2} is rational, or d+b+2 \sqrt{db} is rational, or d+b+2 \sqrt{d} \sqrt{b}

Since \sqrt{d} = e - \sqrt{b}, we can write
d+b+2 \sqrt{d} \sqrt{b} = e^{2}
d+b+2 (e - \sqrt{b}) \sqrt{b} = e^{2}
d + b + 2e \sqrt{b} - b = e^{2}
2 \sqrt{b} = e - d/e

But 2 is rational and \sqrt{b} is irrational so e - d/e must be irrational... a contradiction since d is rational and we made the assumption that e and therefore e^2 was indeed a rational number.

Is this the right idea?

 

[jgens]

That was the general idea that I was trying to get at; however, you can save yourself a little work by considering another square . . .

Suppose that \sqrt{c} + \sqrt{d} = e where e \in \mathbb{Q} and consider \sqrt{c} = e - \sqrt{d}. Clearly c = e^2 - 2e\sqrt{d} + d = (e^2 + d) - 2e\sqrt{d}. Since the first term is rational and the second term necessarily irrational, their sum must also be irrational, contradicting the fact that c \in \mathbb{Q}.

This is the exact proof that I was thinking of but it amounts to the same thing as yours, so it doesn't really matter.