In tetrahedron $ABCD,$
\[\angle ADB = \angle ADC = \angle BDC = 90^\circ.\]Also, $x = \sin \angle CAD$ and $y = \sin \angle CBD.$  Express $\cos \angle ACB$ in terms of $x$ and $y.$
Question:
In tetrahedron $ABCD,$
\[\angle ADB = \angle ADC = \angle BDC = 90^\circ.\]Also, $x = \sin \angle CAD$ and $y = \sin \angle CBD.$  Express $\cos \angle ACB$ in terms of $x$ and $y.$
Answer:
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In tetrahedron $ABCD,$
\[\angle ADB = \angle ADC = \angle BDC = 90^\circ.\]Also, $x = \sin \angle CAD$ and $y = \sin \angle CBD.$  Express $\cos \angle ACB$ in terms of $x$ and $y.$
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