Three cylinders

Geometry Level 3

Imagine three identical infinite cylinders, which center lines are the coordinate axes in 3D. How many corners does their finite intersection region have?

14 9 6 12

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1 solution

David Ingerman
Jul 10, 2018

The coordinates of the corners satisfy the system of equations { x 2 + y 2 = R 2 y 2 + z 2 = R 2 z 2 + x 2 = R 2 \begin{cases} x^2+y^2=R^2\\ y^2+z^2=R^2\\ z^2+x^2=R^2 \end{cases} , where R R is the radius of the cylinders. With simple manipulation the solutions of the system are { x = ± R / 2 y = ± R / 2 z = ± R / 2 \begin{cases} x=\pm R/\sqrt{2}\\ y=\pm R/\sqrt{2}\\ z=\pm R/\sqrt{2} \end{cases} . Also, there're 6 6 corners which are intersections of two elipses. So, there're 6 + 8 = 14 6+8=14 corners.

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