# Altitude (triangle)

Altitude (triangle) "Orthocenter" and "Orthocentre" redirect here. For the orthocentric system, see Orthocentric system. The three altitudes of a triangle intersect at the orthocenter, which for an acute triangle is inside the triangle.

In geometry, an altitude of a triangle is a line segment through a vertex and perpendicular to (i.e., forming a right angle with) a line containing the base (the side opposite the vertex). 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 (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. 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 (as in height), often subscripted with the name of the side the altitude is drawn to.

The altitude of a right triangle from its right angle to its hypotenuse is the geometric mean of the lengths of the segments the hypotenuse is split into. Using Pythagoras' theorem on the 3 triangles of sides (p + q, r, s ), (r, p, h ) and (s, h, q ), {displaystyle {begin{aligned}(p+q)^{2};;&=quad r^{2};;,+quad s^{2}\p^{2}!!+!2pq!+!q^{2}&=overbrace {p^{2}!!+!h^{2}} +overbrace {h^{2}!!+!q^{2}} \2pqquad ;;;&=2h^{2};therefore h!=!{sqrt {pq}}\end{aligned}}} 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 {displaystyle h_{c}={sqrt {pq}}}   (Geometric mean theorem) In a right triangle, the altitude from each acute angle coincides with a leg and intersects the opposite side at (has its foot at) the right-angled vertex, which is the orthocenter. The altitudes from each of the acute angles of an obtuse triangle lie entirely outside the triangle, as does the orthocenter H.

For acute triangles the feet of the altitudes all fall on the triangle's sides (not extended). In an obtuse triangle (one with an obtuse angle), 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.

Contents 1 Orthocenter 1.1 Properties 1.2 Relation with circles and conics 1.3 Relation to other centers, the nine-point circle 2 Orthic triangle 3 Some additional altitude theorems 3.1 Altitude in terms of the sides 3.2 Inradius theorems 3.3 Circumradius theorem 3.4 Interior point 3.5 Area theorem 3.6 General point on an altitude 3.7 Special case triangles 3.7.1 Equilateral triangle 3.7.2 Right triangle 4 History 5 See also 6 Notes 7 References 8 External links Orthocenter See also: Orthocentric system Three altitudes intersecting at the orthocenter 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] Let A, B, C denote the vertices and also the angles of the triangle, and let a = |BC|, b = |CA|, c = |AB| be the side lengths. The orthocenter has trilinear coordinates[3] {displaystyle sec A:sec B:sec C=cos A-sin Bsin C:cos B-sin Csin A:cos C-sin Asin B,} and barycentric coordinates {displaystyle displaystyle (a^{2}+b^{2}-c^{2})(a^{2}-b^{2}+c^{2}):(a^{2}+b^{2}-c^{2})(-a^{2}+b^{2}+c^{2}):(a^{2}-b^{2}+c^{2})(-a^{2}+b^{2}+c^{2})} {displaystyle =tan A:tan B:tan C.} 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.

The extended sides of the orthic triangle meet the opposite extended sides of its reference triangle at three collinear points.[22][23][21] In any acute triangle, the inscribed triangle with the smallest perimeter is the orthic triangle.[24] This is the solution to Fagnano's problem, posed in 1775.[25] The sides of the orthic triangle are parallel to the tangents to the circumcircle at the original triangle's vertices.[26] The orthic triangle of an acute triangle gives a triangular light route.[27] The tangent lines of the nine-point circle at the midpoints of the sides of ABC are parallel to the sides of the orthic triangle, forming a triangle similar to the orthic triangle.[28] The orthic triangle is closely related to the tangential triangle, constructed as follows: let LA be the line tangent to the circumcircle of triangle ABC at vertex A, and define LB and LC analogously. Let A" = LB ∩ LC, B" = LC ∩ LA, C" = LC ∩ LA. The tangential triangle is A"B"C", whose sides are the tangents to triangle ABC'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.[20]: p. 447  Trilinear coordinates for the vertices of the tangential triangle are given by A" = −a : b : c B" = a : −b : c C" = a : b : −c.

The reference triangle and its orthic triangle are orthologic triangles.

Some additional altitude theorems Altitude in terms of the sides For any triangle with sides a, b, c and semiperimeter s = (a + b + c) / 2, the altitude from side a is given by {displaystyle h_{a}={frac {2{sqrt {s(s-a)(s-b)(s-c)}}}{a}}.} This follows from combining Heron's formula for the area of a triangle in terms of the sides with the area formula (1/2)×base×height, where the base is taken as side a and the height is the altitude from A.

Inradius theorems Consider an arbitrary triangle with sides a, b, c and with corresponding altitudes ha, hb, and hc. The altitudes and the incircle radius r are related by[29]: Lemma 1  {displaystyle displaystyle {frac {1}{r}}={frac {1}{h_{a}}}+{frac {1}{h_{b}}}+{frac {1}{h_{c}}}.} Circumradius theorem Denoting the altitude from one side of a triangle as ha, the other two sides as b and c, and the triangle's circumradius (radius of the triangle's circumscribed circle) as R, the altitude is given by[30] {displaystyle h_{a}={frac {bc}{2R}}.} Interior point If p1, p2, and p3 are the perpendicular distances from any point P to the sides, and h1, h2, and h3 are the altitudes to the respective sides, then[31] {displaystyle {frac {p_{1}}{h_{1}}}+{frac {p_{2}}{h_{2}}}+{frac {p_{3}}{h_{3}}}=1.} Area theorem Denoting the altitudes of any triangle from sides a, b, and c respectively as {displaystyle h_{a}} , {displaystyle h_{b}} , and {displaystyle h_{c}} , and denoting the semi-sum of the reciprocals of the altitudes as {displaystyle H=(h_{a}^{-1}+h_{b}^{-1}+h_{c}^{-1})/2} we have[32] {displaystyle mathrm {Area} ^{-1}=4{sqrt {H(H-h_{a}^{-1})(H-h_{b}^{-1})(H-h_{c}^{-1})}}.} General point on an altitude If E is any point on an altitude AD of any triangle ABC, then[33]: 77–78  {displaystyle AC^{2}+EB^{2}=AB^{2}+CE^{2}.} Special case triangles Equilateral triangle 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. This is Viviani's theorem.

Right triangle Comparison of the inverse Pythagorean theorem with the Pythagorean theorem In a right triangle the three altitudes ha, hb, and hc (the first two of which equal the leg lengths b and a respectively) are related according to[34][35] {displaystyle {frac {1}{h_{a}^{2}}}+{frac {1}{h_{b}^{2}}}={frac {1}{h_{c}^{2}}}.} This is also known as the inverse Pythagorean theorem.

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.[36] See also Triangle center Median (geometry) Notes ^ Smart 1998, p. 156 ^ Jump up to: a b Berele & Goldman 2001, p. 118 ^ Clark Kimberling's Encyclopedia of Triangle Centers "Archived copy". Archived from the original on 2012-04-19. Retrieved 2012-04-19. ^ Andreescu, Titu; Andrica, Dorin, "Complex numbers from A to...Z". Birkhäuser, Boston, 2006, ISBN 978-0-8176-4326-3, page 90, Proposition 3 ^ Dörrie, Heinrich, "100 Great Problems of Elementary Mathematics. Their History and Solution". Dover Publications, Inc., New York, 1965, ISBN 0-486-61348-8, page 142 ^ Johnson 2007, p. 163, Section 255 ^ Jump up to: a b ""Orthocenter of a triangle"". Archived from the original on 2012-07-05. Retrieved 2012-05-04. ^ Johnson 2007, p. 176, Section 278 ^ Jump up to: a b Panapoi,Ronnachai, "Some properties of the orthocenter of a triangle", University of Georgia. ^ Smart 1998, p. 182 ^ Weisstein, Eric W. "Isotomic conjugate" From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/IsotomicConjugate.html ^ Weisstein, Eric W. "Orthocenter." From MathWorld--A Wolfram Web Resource. ^ Altshiller-Court 2007, p. 102 ^ Bell, Amy, "Hansen's right triangle theorem, its converse and a generalization", Forum Geometricorum 6, 2006, 335–342. ^ Weisstein, Eric W. "Kiepert Parabola." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/KiepertParabola.html ^ Weisstein, Eric W. "Jerabek Hyperbola." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/JerabekHyperbola.html ^ Berele & Goldman 2001, p. 123 ^ Berele & Goldman 2001, pp. 124-126 ^ Marie-Nicole Gras, "Distances between the circumcenter of the extouch triangle and the classical centers", Forum Geometricorum 14 (2014), 51-61. http://forumgeom.fau.edu/FG2014volume14/FG201405index.html ^ Jump up to: a b Smith, Geoff, and Leversha, Gerry, "Euler and triangle geometry", Mathematical Gazette 91, November 2007, 436–452. ^ Jump up to: a b William H. Barker, Roger Howe (2007). "§ VI.2: The classical coincidences". Continuous symmetry: from Euclid to Klein. American Mathematical Society. p. 292. ISBN 0-8218-3900-4. See also: Corollary 5.5, p. 318. ^ Johnson 2007, p. 199, Section 315 ^ Altshiller-Court 2007, p. 165 ^ Johnson 2007, p. 168, Section 264 ^ Berele & Goldman 2001, pp. 120-122 ^ Johnson 2007, p. 172, Section 270c ^ Bryant, V., and Bradley, H., "Triangular Light Routes," Mathematical Gazette 82, July 1998, 298-299. ^ Kay, David C. (1993), College Geometry / A Discovery Approach, HarperCollins, p. 6, ISBN 0-06-500006-4 ^ Dorin Andrica and Dan S ̧tefan Marinescu. "New Interpolation Inequalities to Euler’s R ≥ 2r". Forum Geometricorum, Volume 17 (2017), pp. 149–156. http://forumgeom.fau.edu/FG2017volume17/FG201719.pdf ^ Johnson 2007, p. 71, Section 101a ^ Johnson 2007, p. 74, Section 103c ^ Mitchell, Douglas W., "A Heron-type formula for the reciprocal area of a triangle," Mathematical Gazette 89, November 2005, 494. ^ Alfred S. Posamentier and Charles T. Salkind, Challenging Problems in Geometry, Dover Publishing Co., second revised edition, 1996. ^ Voles, Roger, "Integer solutions of {displaystyle a^{-2}+b^{-2}=d^{-2}} ," Mathematical Gazette 83, July 1999, 269–271. ^ Richinick, Jennifer, "The upside-down Pythagorean Theorem," Mathematical Gazette 92, July 2008, 313–317. ^ Bogomolny, Alexander, "A Possibly First Proof of the Concurrence of Altitudes", Cut The Knot, retrieved 2019-11-17 References Altshiller-Court, Nathan (2007) [1952], College Geometry, Dover Publications Berele, Allan; Goldman, Jerry (2001), Geometry / Theorems and Constructions, Prentice Hall, ISBN 0-13-087121-4 Johnson, Roger A. (2007) [1960], Advanced Euclidean Geometry, Dover, ISBN 978-0-486-46237-0 Smart, James R. (1998), Modern Geometries (5th ed.), Brooks/Cole, ISBN 0-534-35188-3 External links Weisstein, Eric W. "Altitude". MathWorld. Orthocenter of a triangle With interactive animation Animated demonstration of orthocenter construction Compass and straightedge. Fagnano's Problem by Jay Warendorff, Wolfram Demonstrations Project. Categories: Straight lines defined for a triangle

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