A section of a cylinder with a plane parallel to its axis is a rectangle.
An axial section is a section that passes through the axis of the cylinder.
Theorem A plane parallel to the plane of the base of the cylinder intersects its side surface in a circle equal to the circumference of the base.
Evidence Let α be a plane parallel to the plane of the base of the cylinder. Parallel transfer in the direction of the axis of the cylinder, combines the plane α with the plane of the base of the cylinder, combines the cross-section of the side surface with the plane α with the circumference of the base. The theorem is proved.
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