Two lines, vectors, planes, etc., are said to be perpendicular if they meet at a right angle. In , two vectors and are perpendicular if their dot product
(1)
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In , a line with slope is perpendicular to a line with slope . Perpendicular objects are sometimes said to be "orthogonal."
In the above figure, the line segment is perpendicular to the line segment . This relationship is commonly denoted with a small square at the vertex where perpendicular objects meet, as shown above, and is denoted .
Two trilinear lines
(2)
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(3)
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are perpendicular if
(4)
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(Kimberling 1998, p. 29).