How to Solve a System of Equations with Square Roots?

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Discussion Overview

The discussion revolves around solving a system of equations involving square roots, specifically two equations that relate the variables \(x\) and \(y\). The focus is on finding real solutions to these equations, which include both algebraic manipulation and reasoning about the signs of the variables.

Discussion Character

  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • Some participants present the system of equations multiple times, indicating a focus on clarity in the problem statement.
  • One participant attempts to analyze the positivity of \(x\) and \(y\), concluding that both must be positive based on the properties of the equations.
  • A participant introduces a substitution \(u = \sqrt{x}\) and \(v = \sqrt{y}\) to simplify the equations, leading to a derived condition that results in specific values for \(x\) and \(y\).
  • The derived equations suggest that \(u - \frac{3}{u} = 0\) and \(v - \frac{2}{v} = 0\), which leads to the conclusion that \(x = 3\) and \(y = 2\).

Areas of Agreement / Disagreement

There is no explicit consensus on the solution, as the discussion primarily features one participant's approach and conclusion without further challenges or alternative solutions presented.

Contextual Notes

The discussion does not address potential limitations or assumptions in the approach taken, such as the implications of the positivity condition or the validity of the derived equations across all possible values of \(x\) and \(y\).

Albert1
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$x,y\in R$
$find \,\ solution \,\,of \,\,x\,\, and\,\, y:$
$\left\{\begin{matrix}
x+y+\dfrac{9}{x}+\dfrac{4}{y}=10---(1)\\
(x^2+9)(y^2+4)=24xy---(2)
\end{matrix}\right.$
 
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Albert said:
$x,y\in R$
$find \,\ solution \,\,of \,\,x\,\, and\,\, y:$
$\left\{\begin{matrix}
x+y+\dfrac{9}{x}+\dfrac{4}{y}=10---(1)\\
(x^2+9)(y^2+4)=24xy---(2)
\end{matrix}\right.$
One real solution: x=3, y=2
 
My attempt:

From (2), we get ($x,y \ne 0$):\[xy\left ( x+\frac{9}{x} \right )\left ( y+\frac{4}{y} \right )=24xy\]

or

\[\left ( x+\frac{9}{x} \right )\left ( y+\frac{4}{y} \right )=24\].Let $a = x+\frac{9}{x}$ and $b = y+\frac{4}{y}$. Then the two equations read:\[a + b = 10\: \: \wedge \: \: ab = 24\]So either $a = 4$ and $b = 6$ or vice versa.Case I.
$a = 4$: The corresponding quadratic expression: $x^2-4x+9 = 0$, has no real roots and it makes no sense
to search for a y-root.Case II.
$a = 6$: The corresponding quadratic expression:$ x^2-6x+9 = (x-3)^2$.
$b = 4$: $y^2-4y+4 = (y-2)^2$.

Thus the only solution is the pair: $(x,y) = (3,2)$.
 
Albert said:
$x,y\in R$
$find \,\ solution \,\,of \,\,x\,\, and\,\, y:$
$\left\{\begin{matrix}
x+y+\dfrac{9}{x}+\dfrac{4}{y}=10---(1)\\
(x^2+9)(y^2+4)=24xy---(2)
\end{matrix}\right.$
[sp]The left side of (2) is positive, so $x$ and $y$ are either both positive or both negative. If they are both negative then (1) cannot hold. So they are both positive.

Let $u = \sqrt x$, $v = \sqrt y.$ Then $$\Bigl(u - \dfrac3u\Bigr)^2 + \Bigl(v - \dfrac2v\Bigr)^2 = u^2 - 6 + \frac9{u^2} + v^2 - 4 + \frac4{v^2} = x+y+\dfrac{9}{x}+\dfrac{4}{y}-10 = 0.$$ Therefore $u - \dfrac3u = v - \dfrac2v = 0$, so that $x = u^2 = 3$ and $y = v^2 = 2.$[/sp]
 
Last edited:

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