Perpendicular
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Two lines and are said to be perpendicular if they intersect in right angles. We denote this relationship by . In the Cartesian coordinate system, a line with slope is perpendicular to every line with slope and no others.
One can also discuss perpendicularity of other objects. If a line intersects a plane at a point , we say that if and only if for every line in passing through , .
If a plane intersects another plane in a line , we say that if and only if: for line and passing through , and implies .