Incenter circumcenter orthocenter and centroid of a triangle formula

This wiki page shows some simple examples to solve triangle centers using simple properties like circumcenter, Fermat point, Brocard points, incenter, centroid, orthocenter, etc.

One should be able to recall definitions like

  • circumcenter O,O, the point of which is equidistant from all the vertices of the triangle;
  • incenter I,I, the point of which is equidistant from the sides of the triangle;
  • orthocenter H,H, the point at which all the altitudes of the triangle intersect;
  • centroid G,G, the point of intersection of the medians of the triangle.

An important relationship between these points is the Euler line, which states that O,G,H O, G, H is a straight line and OG:GH=1:2 OG : GH = 1 : 2 . In fact, the center of the Nine Point Circle is also the midpoint of OH OH .

Show that if the orthocenter and the incenter of a triangle coincide, then this triangle must be equilateral.


Consider vertex AA. Let these points coincide at PP.
Then, we know that AP AP is the angle bisector of ∠BAC \angle BAC , and it is also the perpendicular to BC BC . Thus, we obtain that ∠ABC=90∘−∠BAP=90∘−∠CAP=∠ACB \angle ABC = 90 ^ \circ - \angle BAP = 90 ^ \circ - \angle CAP = \angle ACB .

Since this is true from any vertex, it means that all three angles are equal, and thus we have an equilateral triangle.

Let IAIA, IBIB, ICIC denote the distance between the incenter and the vertices of triangle A,B,CA,B,C respectively. Prove that IA⋅IBCA⋅CB+IA⋅ICBA⋅BC+IB⋅ICAB⋅A C=1\frac{IA \cdot IB}{CA \cdot CB} + \frac{IA \cdot IC}{BA \cdot BC} + \frac{IB \cdot IC}{AB \cdot AC} = 1 .


Solution: (to be continued)

Try this:

Prove the Euler line, which states that O,G,H O, G, H is collinear.

Incenter circumcenter orthocenter and centroid of a triangle formula

In triangle ABCABC with centroid GG, if AG=BC AG=BC , what is angle BGCBGC in degrees?

Note: Diagram is not drawn to scale.

How many triangles ABCABC are there with integer side lengths such that the area of the triangle formed by joining the orthocenter, the circumcenter and the centroid of △A BC\triangle ABC is 4444 square units?


Details and assumptions:

The orthocenter of ABCABC is the point at which the altitudes of ABCABC intersect.

The circumcenter of ABCABC is the point which is equidistant from AA, BB and CC.

The centroid of ABC ABC is the point at which the medians of ABCABC intersect.

Every triangle has three “centers” — an incenter, a circumcenter, and an orthocenter — that are located at the intersection of rays, lines, and segments associated with the triangle:

  • Incenter: Where a triangle’s three angle bisectors intersect (an angle bisector is a ray that cuts an angle in half); the incenter is the center of a circle inscribed in (drawn inside) the triangle.

  • Circumcenter: Where the three perpendicular bisectors of the sides of a triangle intersect (a perpendicular bisector is a line that forms a 90° angle with a segment and cuts the segment in half); the circumcenter is the center of a circle circumscribed about (drawn around) the triangle.

  • Orthocenter: Where the triangle’s three altitudes intersect. The altitude of a triangle is a segment from a vertex of the triangle to the opposite side (or to the extension of the opposite side if necessary) that’s perpendicular to the opposite side; the opposite side is called the base.

Finding the incenter

You find a triangle’s incenter at the intersection of the triangle’s three angle bisectors. This location gives the incenter an interesting property: The incenter is equally far away from the triangle’s three sides. No other point has this quality. Incenters, like centroids, are always inside their triangles.

Incenter circumcenter orthocenter and centroid of a triangle formula

The above figure shows two triangles with their incenters and inscribed circles, or incircles (circles drawn inside the triangles so the circles barely touch the sides of each triangle). The incenters are the centers of the incircles. (Don’t talk about this “in” stuff too much if you want to be in with the in-crowd.)

Finding the circumcenter

You find a triangle’s circumcenter at the intersection of the perpendicular bisectors of the triangle’s sides. This location gives the circumcenter an interesting property: the circumcenter is equally far away from the triangle’s three vertices.

Incenter circumcenter orthocenter and centroid of a triangle formula

The above figure shows two triangles with their circumcenters and circumscribed circles, or circumcircles (circles drawn around the triangles so that the circles go through each triangle’s vertices). The circumcenters are the centers of the circumcircles.

You can see in the above figure that, unlike centroids and incenters, a circumcenter is sometimes outside the triangle. The circumcenter is

  • Inside all acute triangles

  • Outside all obtuse triangles

  • On all right triangles (at the midpoint of the hypotenuse)

Finding the orthocenter

Check out the following figure to see a couple of orthocenters. You find a triangle’s orthocenter at the intersection of its altitudes. Unlike the centroid, incenter, and circumcenter — all of which are located at an interesting point of the triangle (the triangle’s center of gravity, the point equidistant from the triangle’s sides, and the point equidistant from the triangle’s vertices, respectively), a triangle’s orthocenter doesn’t lie at a point with any such nice characteristics. Well, three out of four ain’t bad.

Incenter circumcenter orthocenter and centroid of a triangle formula

But get a load of this: Look again at the triangles in the figure. Take the four labeled points of either triangle (the three vertices plus the orthocenter). If you make a triangle out of any three of those four points, the fourth point is the orthocenter of that triangle. Pretty sweet, eh?

Orthocenters follow the same rule as circumcenters (note that both orthocenters and circumcenters involve perpendicular lines — altitudes and perpendicular bisectors): The orthocenter is

  • Inside all acute triangles

  • Outside all obtuse triangles

  • On all right triangles (at the right angle vertex)

About This Article

This article can be found in the category:

  • Geometry ,

What is incenter circumcenter orthocenter centroid?

circumcenter O, the point of which is equidistant from all the vertices of the triangle; incenter I, the point of which is equidistant from the sides of the triangle; orthocenter H, the point at which all the altitudes of the triangle intersect; centroid G, the point of intersection of the medians of the triangle.

What is incentre formula?

Incenter of a Triangle Properties If I is the incenter of the triangle ABC, then ∠BAI = ∠CAI, ∠BCI = ∠ACI and ∠ABI = ∠CBI (using angle bisector theorem). The sides of the triangle are tangents to the circle, and thus, EI = FI = GI = r known as the inradii of the circle or radius of incircle.

What are the 4 centers of triangles and how are they constructed?

Triangle Centers. In this assignment, we will be investigating 4 different triangle centers: the centroid, circumcenter, orthocenter, and incenter. The centroid of a triangle is constructed by taking any given triangle and connecting the midpoints of each leg of the triangle to the opposite vertex.