In Euclidean geometry, a plane is defined by:
- three non-collinear points
- a line and a point not on the line
- two intersecting lines
- two parallel lines
A line can intersect a plane in a single point, or it can be in the plane.
Two planes perpendicular to the same line are parallel to each other.
Two lines perpendicular to the same plane are parallel to each other.
Two planes can be parallel, or they can intersect in a line.
If two lines are perpendicular to each other and one of them is in a plane, then the other is either also in that plane or perpendicular to the plane.
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