# Math Help - Prove perfect squares

1. ## Prove perfect squares

$\sqrt{p} +\sqrt{q} +\sqrt{z} =s$

Prove that p, q and r must be perfect squares.

Ok so the rule for a square is: $n^2=(n-1)^2+\sqrt{(2n-1)}$

So $p=p-1+\sqrt{2p-1}$ then $p-p=-1+\sqrt{(2p-1)}$ therefore if you square it $0=1+2p-1$ which goes to $0=2p$

Where did I go wrong?

2. I think you went wrong in your first step.

If you substitute $n = \sqrt{p}$ into $n^2 = (n-1)^2 + \sqrt{2n-1}$ you get:

$p = (\sqrt{p}-1)^2 + \sqrt{2 \sqrt{p} - 1}$ and not what you wrote

Another thing is that

$(-1 + \sqrt{2p-1})^2 \neq 1 + 2p-1$
it doest equal $(-1 + \sqrt{2p-1})(-1 + \sqrt{2p-1})$, where you multiply out the brackets.

3. Originally Posted by Mukilab
Ok so the rule for a square is: $n^2=(n-1)^2+\sqrt{(2n-1)}$
Where did you get this? If you plug in $n=2$, the right hand side yields $1+\sqrt{3}$ instead of $4$.

From context it's clear that you mean $p, q, r, s \in \mathbb{Z}$.

Then it's a matter of proving that if p, q and r are not all perfect squares, then s cannot be rational.

4. That's the formula for proving the square of a number using the previous number squared.

5. I don't know if it's a matter of whether p, q and r are perfect squares or not.

Never mind, this question does not apply to me anymore and I have far more tricky questions to get around :P

6. Originally Posted by Mukilab
That's the formula for proving the square of a number using the previous number squared.
Well just for reference,

$(n-1)^2=n^2-2n+1$

$n^2=(n-1)^2+2n-1$