2005 Canadian MO Problems/Problem 2

Problem

Let $(a,b,c)$ be a Pythagorean triple, i.e., a triplet of positive integers with ${a}^2+{b}^2={c}^2$.

  • Prove that $(c/a + c/b)^2 > 8$.
  • Prove that there does not exist any integer $n$ for which we can find a Pythagorean triple $(a,b,c)$ satisfying $(c/a + c/b)^2 = n$.

Solution

  • We have
$\left(\frac ca + \frac cb\right)^2 = \frac{c^2}{a^2} + 2\frac{c^2}{ab} + \frac{c^2}{b^2} = \frac{a^2 + b^2}{a^2} + 2\frac{a^2 + b^2}{ab} + \frac{a^2+b^2}{b^2} = 2 + \left(\frac{a^2}{b^2} + \frac{b^2}{a^2}\right) + 2\left(\frac ab + \frac ba\right)$

By AM-GM, we have

$x + \frac 1x > 2,$

where $x$ is a positive real number not equal to one. If $a = b$, then $c \not\in \mathbb{Z}$. Thus $a \neq b$ and $\frac ab \neq 1\implies \frac{a^2}{b^2}\neq 1$. Therefore,

$\left(\frac ca + \frac cb\right)^2 > 2 + 2 + 2(2) = 8.$
  • Now since $a$, $b$, and $c$ are positive integers, $c/a + c/b$ is a rational number $p/q$, where $p$ and $q$ are positive integers. Now if $p^2/q^2=n$, where $n$ is an integer, then $p/q$ must also be an integer. Thus $c(a+b)/ab$ must be an integer.

Now every pythagorean triple can be written in the form $(2mn, m^2-n^2, m^2+n^2)$, with $m$ and $n$ positive integers. Thus one of $a$ or $b$ must be even. If $a$ and $b$ are both even, then $c$ is even too. Factors of 4 can be cancelled from the numerator and the denominator(since every time one of $a$, $b$, $c$, and $a+b$ increase by a factor of 2, they all increase by a factor of 2) repeatedly until one of $a$, $b$, or $c$ is odd, and we can continue from there. Thus the $m^2-n^2$ term is odd, and thus $c$ is odd. Now $c$ and $a+b$ are odd, and $ab$ is even. Thus $c(a+b)/ab$ is not an integer. Now we have reached a contradiction, and thus there does not exist any integer $n$ for which we can find a Pythagorean triple $(a,b,c)$ satisfying $(c/a + c/b)^2 = n$.

See also

2005 Canadian MO (Problems)
Preceded by
Problem 1
1 2 3 4 5 Followed by
Problem 3
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