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In geometry, an altitude of a triangle is a straight line through a vertex and perpendicular to (i.e. forming a right angle with) the opposite side or an extension of the opposite side. The intersection between the (extended) side and the altitude is called the foot of the altitude. This opposite side is called the base of the altitude. The length of the altitude is the distance between the base and the vertex.

In an isosceles triangle (a triangle with two congruent sides), the altitude having the incongruent side as its base will have the midpoint of that side as its foot.

Altitudes can be used to compute 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. Through trigonometric functions, it can also give the length of one side of the triangle.

In a right triangle, the altitude with the hypotenuse as base divides the hypotenuse into two lengths p and q. If we denote the length of the altitude by h, we then have the relation

h² = pq.

Contents 1 The orthocenter 2 Orthic triangle 3 Some additional altitude theorems 3.1 Inradius theorems 3.2 The symphonic theorem* 4 External links

The three altitudes intersect in a single point, called the orthocenter of the triangle. The orthocenter lies inside the triangle (and consequently the feet of the altitudes all fall on the triangle) if and only if the triangle is not obtuse (i.e. does not have an angle greater than a right angle). See also orthocentric system.

The orthocenter, along with the centroid, circumcenter and 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 between the orthocenter and the circumcenter, and the distance between the centroid and the circumcenter is half that between the centroid and the orthocenter.

The isogonal conjugate of the orthocenter is the circumcenter.

Four points in the plane such that one of them is the orthocenter of the triangle formed by the other three are called an orthocentric system or orthocentric quadrangle.

Let A, B, C denote the angles of the reference triangle, and let a = |BC|, b = |CA|, c = |AB| be the sidelengths. The orthocenter has trilinear coordinates sec A : sec B : sec C and barycentric coordinates

((a² + b² − c²)( − a² − b² + c²):(a² + b² − c²)( − a² + b² + c²):(a² − b² + c²)( − a² + b² + c²)).

The points of intersection of the altitudes with the sides of the triangles form another triangle, A'B'C', called the orthic triangle or altitude triangle. It is the pedal triangle of the orthocenter of the original triangle. Also, the incenter of the orthic triangle is the orthocenter of the original triangle.

The orthic triangle is closely related to the tangential triangle, constructed as follows: let L_A be the line tangent to the circumcircle of triangle ABC at vertex A, and define L_B and L_C analogously. Let A" = L_B ∩ L_C, B" = L_C ∩ L_A, C" = L_C ∩ L_A. The tangential triangle, A"B"C", is homothetic to the orthic triangle.

The orthic triangle provides the solution to Fagnano's problem which in 1775 asked for the minimum perimeter triangle inscribed in a given acute-angle triangle.

Trilinear coordinates for the vertices of the orthic triangle are given by

• A' = 0 : sec B : sec C
• B' = sec A : 0 : sec C
• C' = sec A : sec B : 0

Trilinear coordinates for the vertices of the tangential triangle are given by

• A" = −a : b : c
• B" = a : −b : c
• C" = a : b : −c

Equilateral triangle theorem:

For any point P within an equilateral triangle, the sum of the perpendiculars to the three sides is equal to the altitude of the triangle.

Consider an arbitrary triangle with sides a, b, c and with corresponding altitudes α, β, η. The altitudes and incircle radius r are related by

Let c, h, s be the sides of 3 squares associated with the right triangle; the square on the hypotenuse, and the triangle's 2 inscribed squares respectively. The sides of these squares (c>h>s) and the incircle radius r are related by a similar formula:

In the case of the right triangle, the sides of the 3 squares c, h, s are related to each other by the symphonic theorem, as are the 3 altitudes α, β, η. The symphonic theorem states that triples (c²,h²,s²) and (α²,β²,η²) are harmonic, and that triples and are Pythagorean:

• H. Lee Price and Frank R. Bernhart, Pythagorean triples and a new Pythagorean theorem, arxiv.org:math/0701554, (2007) [1],[*symphonic theorem]
• Triangle centers by Antonio Gutierrez from Geometry Step by Step from the Land of the Incas.
• Orthocenter of a triangle With interactive animation
• Animated demonstration of orthocenter construction Compass and straightedge.
• An interactive Java applet for the orthocenter