Quadratic Irrational is Root of Quadratic Equation

Theorem
Let $x$ be a quadratic irrational.

Then $x$ is a solution to a quadratic equation with rational coefficients.

Proof
Let $x = r + s \sqrt n$.

From Solution to Quadratic Equation, the solutions of $a x^2 + b x + c$ are:
 * $x = \dfrac {-b \pm \sqrt {b^2 - 4 a c}} {2 a}$

given the appropriate condition on the discriminant.

So if $x = r + s \sqrt n$ is a solution, then so is $x = r - s \sqrt n$.

Hence we have:

As $r$ and $s$ are rational and $n$ is an integer, it follows that $-2 r$ and $r^2 - s^2 n$ are also rational from Rational Numbers form Field.

Hence the result.