Rotate and fit an rectangle into another rectangle.

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I have the task of fitting a rotated rectangle into another rectangle. Inner rectangle shall touch outer rectangle on each side with one edge of inner rectangle.

Rectangle in rectangle

Given values

$a,b$ : Size of outer rectangle.

$w$ : width of inner rectangle near side a. $w < \sqrt{a^2+b^2}$, so we know it will fit

Unknown values

$x,y$ : section of side $a$ and side $b$, where inner rectangle touches outer rectangle

$z$ : length of inner rectangle (but can easily be calculated, when $x$ and $y$ is known)

What I have tried:

$x^2 + y^2 = w^2$ [pythagoras]

$\frac{x}{y} = \frac{b − y}{a − x}$ [congruent triangles]

Solve equations for $y$

$y = \sqrt{w^2 − x^2}$

$y = b/2 + \sqrt{x^2 + b^2/4 − ax}$

Now I can eliminate $y$

$\sqrt{w^2 − x^2} = b/2 + \sqrt{x^2 + b^2/4 − ax}$

But I am not able to transform this into

$x = f(a,b,w)$