how do yoou construct an altitude

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Mr. McCall explains how to construct a perpendicular line to a segment through a point in this video:http://www.brightstorm.com/d/math/s/geometry/u/constructions/t/constructing-a-perpendicular-to-a-lineJust use the triangle base as your line segment, and the vertex opposite the base as your point P, and use the same methods to construct the perpendicular, or altitude.