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If $x^2-bx+a = 0$ and $x^2-ax+b = 0$ both have distinct positive integers roots, then $(a,b)$ is $______$?

My Try:: for first :: $\displaystyle x^2-ax+b = 0\Rightarrow x = \frac{a\pm \sqrt{a^2-4b}}{2}$

So Here $(a^2-4b)\in $ perfect square.

Similarly $\displaystyle x^2-bx+a = 0\Rightarrow x = \frac{b\pm \sqrt{b^2-4a}}{2}$

So Here $(b^2-4a)\in $ perfect square.

But I did not understand how can i solve after that.

plz help me

Thanks

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2  
Then (a,b) is what? –  Ian Coley Sep 15 '13 at 19:39
    
I mean ordered pair $(a,b)$ is –  juantheron Sep 15 '13 at 19:45
    
You don't mean $\gcd$? –  Git Gud Sep 15 '13 at 19:46
    
Yes Git Gud all ordered pair $(a,b)$ –  juantheron Sep 15 '13 at 19:48
1  
Git Gud is not referring to an ordered pair. Git Gud is asking if you are using the symbol, $(a,b)$, to mean the gcd (greatest common divisor) of $a$ and $b$. –  user71352 Sep 15 '13 at 19:52

3 Answers 3

up vote 7 down vote accepted

Let the roots of $x^2 - ax +b$ be $r$ and $s$, and the roots of $x^2 - bx + a$ be $u$ and $v$. Then

$$\begin{align} r+s &= a\\ rs &= b\\ u+v &= b\\ uv &= a. \end{align}$$

Let, without loss of generality, $a \leqslant b$. Thus

$$\begin{align} uv &\leqslant u+v\\ \iff uv - u - v + 1 & \leqslant 1\\ \iff (u-1)(v-1) & \leqslant 1. \end{align}$$

So $u = 1$ or $v = 1$ or $u = v = 2$. But the roots are supposed to be distinct, hence $u = 1$ or $v = 1$. Without loss of generality, $u = 1$.

Thus $a = v = r+s$, $b = v+1 = rs$, so

$$rs = r+s+1 \iff (r-1)(s-1) = 2,$$

and that leaves $r = 2$, $s = 3$ (or vice versa), so $a = 5, b = 6$.

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Suppose $x^2-ax+b$ and $x^2-bx+a$ have positive integer roots. Then $$x^2-ax+b=(x-c)(x-d)\qquad\text{and}\qquad x^2-bx+a=(x-e)(x-f),$$ for some positive integers $c$, $d$, $e$ and $f$. As the roots are must be distinct, we may assume without loss of generality that $c>d$ and $e>f$. Now compare coefficients.

Hint 1:

It follows that $c+d=a=ef$ and $cd=b=e+f$.

Hint 2:

If $c>d>1$ then $cd>c+d$.

Hint 3:

If $d>1$ and $f>1$ then $cd>c+d=ef>e+f=cd$, a contradiction.

Hint 4:

Without loss of generality we have $f=1$, so $e=c+d$ and $e+1=cd$.

Hint 5:

It follows that $(c-1)(d-1)=2$, so $c=3$ and $d=2$, and hence $e=5$.

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Would a=b=4 work? In that case the (double) solution in both cases is 2 and that's positive

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The OP states that the roots are distinct. –  Daryl Sep 15 '13 at 20:05
    
Bummer, couldn't be that easy I guess :) –  imranfat Sep 15 '13 at 20:06

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