where do the altitudes of a triangle intersect
Altitude (triangle) The three (possibly extended) altitudes intersect in a single point, called the orthocenter of the triangle, usually denoted by H.[1][2] The orthocenter lies inside the triangle if and only if the triangle is acute (i.e. does not have an angle greater than or equal to a right angle). If one angle is a right angle, the orthocenter coincides with the vertex at the right angle.[2]
Limit of a function (the Dirichlet function) has no limit at any x-coordinate.
Perpendicular The legs of a right triangle are perpendicular to each other.
Alonso Álvarez de Pineda