1963 IMO Problems/Problem 1

Problem

Find all real roots of the equation

$\sqrt{x^2-p}+2\sqrt{x^2-1}=x$,

where $p$ is a real parameter.

Video Solution

https://youtu.be/N499P8lsb7U?si=BH8letxqFGEVpq34 [Video Solution by little-fermat]


Solution

Assuming $x \geq 0$, square the equation, obtaining \[4\sqrt {(x^2 - p)(x^2 - 1)} = p + 4 - 4x^2\]. If we have $p + 4 \geq 4x^2$, we can square again, obtaining \[x^2 = \frac {(p - 4)^2}{4(4 - 2p)} \implies x = \pm\frac {p - 4}{2\sqrt {4 - 2p}}\]

We must have $4 - 2p > 0 \iff p < 2$, so we have \[x = \frac {4 - p}{2\sqrt {4 - 2p}}\]

However, this is only a solution when \[p + 4 \geq 4x^2 = \frac {(p - 4)^2}{4 - 2p} \iff (p + 4)(4 - 2p)\geq(p - 4)^2 \iff 0\geq p(3p - 4)\]

so we have $p\geq 0$ and $p \leq \frac {4}{3}$

and $x = \frac {4 - p}{2\sqrt {4 - 2p}}$

See Also

1963 IMO (Problems) • Resources
Preceded by
First Question
1 2 3 4 5 6 Followed by
Problem 2
All IMO Problems and Solutions