Naturally, if a line is given, then a perpendicular is any line at a ninetydegree angle to that line. This is an important property in geometry and trigonometry since important properties accrue to line systems containing right angles. When graphing, the convention is to use either an X and Y axis, or to use an X, Y, and Z axis, which are defined as being mutually perpendicular. Right triangles, too, include two perpendicular lines and so have special properties, which are the foundation of trigonometry.
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