Altitude (triangle)


In geometry, an altitude of a triangle is a line segment through a vertex and perpendicular to a line containing the base. This line containing the opposite side is called the extended base of the altitude. The intersection of the extended base and the altitude is called the foot of the altitude. The length of the altitude, often simply called "the altitude", is the distance between the extended base and the vertex. The process of drawing the altitude from the vertex to the foot is known as dropping the altitude at that vertex. It is a special case of orthogonal projection.
Altitudes can be used in the computation of the area of a triangle: one half of the product of an altitude's length and its base's length equals the triangle's area. Thus, the longest altitude is perpendicular to the shortest side of the triangle. The altitudes are also related to the sides of the triangle through the trigonometric functions.
In an isosceles triangle, the altitude having the incongruent side as its base will have the midpoint of that side as its foot. Also the altitude having the incongruent side as its base will be the angle bisector of the vertex angle.
It is common to mark the altitude with the letter h, often subscripted with the name of the side the altitude is drawn to.
In a right triangle, the altitude drawn to the hypotenuse c divides the hypotenuse into two segments of lengths p and q. If we denote the length of the altitude by hc, we then have the relation
For acute and right triangles the feet of the altitudes all fall on the triangle's sides. In an obtuse triangle, the foot of the altitude to the obtuse-angled vertex falls in the interior of the opposite side, but the feet of the altitudes to the acute-angled vertices fall on the opposite extended side, exterior to the triangle. This is illustrated in the adjacent diagram: in this obtuse triangle, an altitude dropped perpendicularly from the top vertex, which has an acute angle, intersects the extended horizontal side outside the triangle.

Orthocenter

The three altitudes intersect in a single point, called the orthocenter of the triangle, usually denoted by. The orthocenter lies inside the triangle if and only if the triangle is acute. If one angle is a right angle, the orthocenter coincides with the vertex at the right angle.
Let denote the vertices and also the angles of the triangle, and let be the side lengths. The orthocenter has trilinear coordinates
and barycentric coordinates
Since barycentric coordinates are all positive for a point in a triangle's interior but at least one is negative for a point in the exterior, and two of the barycentric coordinates are zero for a vertex point, the barycentric coordinates given for the orthocenter show that the orthocenter is in an acute triangle's interior, on the right-angled vertex of a right triangle, and exterior to an obtuse triangle.
In the complex plane, let the points and represent the numbers, and, respectively, and assume that the circumcenter of triangle is located at the origin of the plane. Then, the complex number
is represented by the point, namely the orthocenter of triangle. From this, the following characterizations of the orthocenter by means of free vectors can be established straightforwardly:
The first of the previous vector identities is also known as the problem of Sylvester, proposed by James Joseph Sylvester.

Properties

Let, and denote the feet of the altitudes from, and respectively. Then:
Denote the circumradius of the triangle by. Then
In addition, denoting as the radius of the triangle's incircle,, and as the radii of its excircles, and again as the radius of its circumcircle, the following relations hold regarding the distances of the orthocenter from the vertices:
If any altitude, for example,, is extended to intersect the circumcircle at, so that is a chord of the circumcircle, then the foot bisects segment :
The directrices of all parabolas that are externally tangent to one side of a triangle and tangent to the extensions of the other sides pass through the orthocenter.
A circumconic passing through the orthocenter of a triangle is a rectangular hyperbola.

Relation to other centers, the nine-point circle

The orthocenter, the centroid, the circumcenter, and the center of the nine-point circle all lie on a single line, known as the Euler line. The center of the nine-point circle lies at the midpoint of the Euler line, between the orthocenter and the circumcenter, and the distance between the centroid and the circumcenter is half of that between the centroid and the orthocenter:
The orthocenter is closer to the incenter than it is to the centroid, and the orthocenter is farther than the incenter is from the centroid:
In terms of the sides, inradius and circumradius,

Orthic triangle

If the triangle is oblique, the pedal triangle of the orthocenter of the original triangle is called the orthic triangle or altitude triangle. That is, the feet of the altitudes of an oblique triangle form the orthic triangle,. Also, the incenter of the orthic triangle is the orthocenter of the original triangle.
Trilinear coordinates for the vertices of the orthic triangle are given by
The extended sides of the orthic triangle meet the opposite extended sides of its reference triangle at three collinear points.
In any acute triangle, the inscribed triangle with the smallest perimeter is the orthic triangle. This is the solution to Fagnano's problem, posed in 1775. The sides of the orthic triangle are parallel to the tangents to the circumcircle at the original triangle's vertices.
The orthic triangle of an acute triangle gives a triangular light route.
The tangent lines of the nine-point circle at the midpoints of the sides of are parallel to the sides of the orthic triangle, forming a triangle similar to the orthic triangle.
The orthic triangle is closely related to the tangential triangle, constructed as follows: let be the line tangent to the circumcircle of triangle at vertex, and define and analogously. Let,,. The tangential triangle is , whose sides are the tangents to triangle 's circumcircle at its vertices; it is homothetic to the orthic triangle. The circumcenter of the tangential triangle, and the center of similitude of the orthic and tangential triangles, are on the Euler line.
Trilinear coordinates for the vertices of the tangential triangle are given by
For more information on the orthic triangle, see here.

Some additional altitude theorems

Altitude in terms of the sides

For any triangle with sides and semiperimeter, the altitude from side is given by
This follows from combining Heron's formula for the area of a triangle in terms of the sides with the area formula ×base×height, where the base is taken as side and the height is the altitude from.

Inradius theorems

Consider an arbitrary triangle with sides and with corresponding
altitudes, and. The altitudes and the incircle radius are related by

Circumradius theorem

Denoting the altitude from one side of a triangle as, the other two sides as and, and the triangle's circumradius as, the altitude is given by

Interior point

If, and are the perpendicular distances from any point to the sides, and, and are the altitudes to the respective sides, then

Area theorem

Denoting the altitudes of any triangle from sides, and respectively as,, and, and denoting the semi-sum of the reciprocals of the altitudes as we have

General point on an altitude

If is any point on an altitude of any triangle, then

Special case triangles

Equilateral triangle

For any point within an equilateral triangle, the sum of the perpendiculars to the three sides is equal to the altitude of the triangle. This is Viviani's theorem.

Right triangle

In a right triangle the three altitudes, and are related according to

History

The theorem that the three altitudes of a triangle meet in a single point, the orthocenter, was first proved in a 1749 publication by William Chapple.