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A triangle or trigon is a two dimensional geometric object that has the specific qualities of having three straight sides that intersect at three vertices.

The sum of the internal angles that exist at the vertices always total the same number for every triangle — 180 degrees, or $\pi$ radians.

In Euclidean geometry, any three non-collinear points determine a unique triangle and a unique plane.

## Types of triangles

### By relative lengths of sides

Triangles can be classified according to the relative lengths of their sides:

• In an equilateral triangle, all sides are the same length. An equilateral triangle is also a regular polygon with all angles 60°.
• In an isosceles triangle, at least two sides are equal in length. An isosceles triangle also has two equal angles: the angles opposite the two equal sides.
• In a scalene triangle, all sides and internal angles are different from one another.

### By internal angles

Triangles can also be classified according to their internal angles, measured here in degrees.

A triangle that does not contain a right angle is called an oblique triangle. One that does is a right triangle.

• There are two types of oblique triangles, those with all the internal angles smaller than 90°, and those with one angle larger than 90°.

The obtuse triangle contains the larger than 90° angle, known as an obtuse angle. The acute triangle is composed of three acute angles, the same as saying that all three of its angles are smaller than 90°.

• A right triangle (or right-angled triangle) has one $90^\circ$ internal angle (a right angle). The side opposite to the right angle is the hypotenuse; it is the longest side in the right triangle. Right triangles conform to the Pythagorean theorem: the sum of the squares of the two legs is equal to the square of the hypotenuse; i.e. $a^2+b^2=c^2$ , where $a,b$ are the legs and $c$ is the hypotenuse. See also Special right triangles.
• An equiangular triangle is also an equilateral triangle.

## Basic facts

Triangles are assumed to be two-dimensional plane figures, unless the context provides otherwise (see Non-planar triangles, below). In rigorous treatments, a triangle is therefore called a 2-simplex (see also Polytope). Elementary facts about triangles were presented by Euclid in books 1–4 of his Elements, around 300 BCE.

The internal angles of a triangle in Euclidean space always add up to 180 degrees. This allows determination of the third angle of any triangle as soon as two angles are known. An exterior angle of a triangle is an angle that is adjacent and supplementary to an internal angle. Any external angle of any triangle is equal to the sum of the two internal angles that it is not adjacent to; this is the exterior angle theorem. The three external angles (one for each vertex) of any triangle add up to 360 degrees. (The $n$ external angles of any $n$-sided convex polygon add up to 360 degrees.)

The sum of the lengths of any two sides of a triangle always exceeds the length of the third side, a principle known as the triangle inequality. In a special case, the sum is equal to the length of the third side; but in this case the triangle has arguably degenerated to a line segment, or to a digon.

Two triangles are said to be similar if and only if each internal angle of one triangle is equal to an internal angle of the other. It is not required to specify that the equal angles be corresponding angles, since any triangle is by definition similar to its own "mirror image". In this case, all sides of one triangle are in equal proportion to sides of the other triangle.

A few basic postulates and theorems about similar triangles:

• If two corresponding internal angles of two triangles are equal, the triangles are similar.
• If two corresponding sides of two triangles are in proportion, and their included angles are equal, then the triangles are similar. (The included angle for any two sides of a polygon is the internal angle between those two sides.)
• If three corresponding sides of two triangles are in proportion, then the triangles are similar.

Two triangles that are congruent have exactly the same size and shape:[1] all corresponding internal angles are equal in size, and all corresponding sides are equal in length. (This is a total of six equalities, but three are often sufficient to prove congruence.)

Some sufficient conditions for a pair of triangles to be congruent (from basic postulates and theorems of Euclid):

• SAS Postulate: Two sides and the included angle in a triangle are equal to two sides and the included angle in the other triangle.
• ASA Postulate: Two internal angles and the included side in a triangle are equal to those in the other triangle. (The included side for a pair of angles is the side between them.)
• SSS Postulate: Each side of a triangle is equal in length to a side of the other triangle.
• AAS Theorem: Two angles and a corresponding non-included side in a triangle are equal to those in the other triangle.
• Hypotenuse-Leg (HL) Theorem: The hypotenuse and a leg in a right triangle are equal to those in the other right triangle.
• Hypotenuse-Angle Theorem: The hypotenuse and an acute angle in one right triangle are equal to those in the other right triangle.

An important case:

• Side-Side-Angle (or Angle-Side-Side) condition: If two sides and a corresponding non-included angle of a triangle are equal to those in another, then this is not sufficient to prove congruence; but if the non-included angle is obtuse or a right angle, or the side opposite it is the longest side, or the triangles have corresponding right angles, then the triangles are congruent. The Side-Side-Angle condition does not by itself guarantee that the triangles are congruent because one triangle could be obtuse-angled and the other acute-angled.

Using right triangles and the concept of similarity, the trigonometric functions sine and cosine can be defined. These are functions of an angle which are investigated in trigonometry.

A central theorem is the Pythagorean theorem, which states in any right triangle, the square of the length of the hypotenuse equals the sum of the squares of the lengths of the two other sides. If the hypotenuse has length $c$ , and the legs have lengths $a,b$ , then the theorem states that

$a^2+b^2=c^2$

The converse is true: if the lengths of the sides of a triangle satisfy the above equation, then the triangle is a right triangle.

Some other facts about right triangles:

$a+b+90^\circ=180^\circ\ \Rarr\ a+b=90^\circ\ \Rarr\ a=90^\circ-b$
• If the legs of a right triangle are equal, then the angles opposite the legs are equal, acute, and complementary; each is therefore $45^\circ$ . By the Pythagorean theorem, the length of the hypotenuse is the length of a leg times $\sqrt2$ .
• In a right triangle with acute angles measuring 30 and 60 degrees, the hypotenuse is twice the length of the shorter side, and twice the length divided by $\sqrt3$ for the longer side.

For all triangles, angles and sides are related by the law of cosines and law of sines.

## Points, lines and circles associated with a triangle

There are hundreds of different constructions that find a special point associated with (and often inside) a triangle, satisfying some unique property: see the references section for a catalogue of them. Often they are constructed by finding three lines associated in a symmetrical way with the three sides (or vertices) and then proving that the three lines meet in a single point: an important tool for proving the existence of these is Ceva's theorem, which gives a criterion for determining when three such lines are concurrent. Similarly, lines associated with a triangle are often constructed by proving that three symmetrically constructed points are collinear: here Menelaus' theorem gives a useful general criterion. In this section just a few of the most commonly-encountered constructions are explained.

A perpendicular bisector of a triangle is a straight line passing through the midpoint of a side and being perpendicular to it, i.e. forming a right angle with it. The three perpendicular bisectors meet in a single point, the triangle's circumcenter; this point is the center of the circumcircle, the circle passing through all three vertices. The diameter of this circle can be found from the law of sines stated above.

Thales' theorem implies that if the circumcenter is located on one side of the triangle, then the opposite angle is a right one. More is true: if the circumcenter is located inside the triangle, then the triangle is acute; if the circumcenter is located outside the triangle, then the triangle is obtuse.

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. This opposite side is called the base of the altitude, and the point where the altitude intersects the base (or its extension) is called the foot of the altitude. The length of the altitude is the distance between the base and the vertex. The three altitudes intersect in a single point, called the orthocenter of the triangle. The orthocenter lies inside the triangle if and only if the triangle is acute. The three vertices together with the orthocenter are said to form an orthocentric system.

An angle bisector of a triangle is a straight line through a vertex which cuts the corresponding angle in half. The three angle bisectors intersect in a single point, the incenter, the center of the triangle's incircle. The incircle is the circle which lies inside the triangle and touches all three sides. There are three other important circles, the excircles; they lie outside the triangle and touch one side as well as the extensions of the other two. The centers of the in- and excircles form an orthocentric system.

A median of a triangle is a straight line through a vertex and the midpoint of the opposite side, and divides the triangle into two equal areas. The three medians intersect in a single point, the triangle's centroid. The centroid of a stiff triangular object (cut out of a thin sheet of uniform density) is also its center of gravity: the object can be balanced on its centroid. The centroid cuts every median in the ratio 2:1, i.e. the distance between a vertex and the centroid is twice the distance between the centroid and the midpoint of the opposite side.

The midpoints of the three sides and the feet of the three altitudes all lie on a single circle, the triangle's nine-point circle. The remaining three points for which it is named are the midpoints of the portion of altitude between the vertices and the orthocenter. The radius of the nine-point circle is half that of the circumcircle. It touches the incircle (at the Feuerbach point) and the three excircles.

The centroid (yellow), orthocenter (blue), circumcenter (green) and barycenter of the nine-point circle (red point) all lie on a single line, known as Euler's line (red 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 center of the incircle is not in general located on Euler's line.

If one reflects a median at the angle bisector that passes through the same vertex, one obtains a symmedian. The three symmedians intersect in a single point, the symmedian point of the triangle.

## Computing the area of a triangle

Calculating the area of a triangle is an elementary problem encountered often in many different situations. The best known and simplest formula is:

$A=\frac{bh}{2}$

where $A$ is area, $b$ is the length of the base of the triangle, and $h$ is the height or altitude of the triangle. The term 'base' denotes any side, and 'height' denotes the length of a perpendicular from the point opposite the side onto the side itself.

The area of a triangle is the area of any quadrilateral divided by 2, therefore the following formulas are:

\begin{align}A&=\frac{A_4}{2}\\A&=\frac{h(b_1+b_2)}{4}\\A&=\frac{\sqrt{abcd}}{2}\\A&=\frac{S^2}{2}\\A&=\frac{(a+c)\sqrt{(a+b-c+d)(a-b-c+d)(a+b-c-d)(b-a+c+d)}}{8(a-c)}\end{align}

Although simple, this formula is only useful if the height can be readily found. For example, the surveyor of a triangular field measures the length of each side, and can find the area from his results without having to construct a 'height'. Various methods may be used in practice, depending on what is known about the triangle. The following is a selection of frequently used formulae for the area of a triangle.

Another way to find the area of a triangle:

$A=\frac{b\sqrt{4a^2-b^2}}{4}$

The area of a triangle is the area of any $n$-sided polygon divided by $n-2$ therefore the formula is:

$A=\frac{A_n}{n-2}=\frac{n}{4(n-2)\tan\left(\frac{180}{n}\right)}s^2$

### Using vectors

The area of a parallelogram can be calculated using vectors. Let vectors $\mathbf{AB},\mathbf{AC}$ point respectively from $A$ to $B$ and from $A$ to $C$ . The area of parallelogram ABDC is then $|\mathbf{AB}\times\mathbf{AC}|$ , which is the magnitude of the cross product of vectors $\mathbf{AB},\mathbf{AC}$ . $|\mathbf{AB}\times\mathbf{AC}|$ is equal to $|\mathbf{h}\times\mathbf{AC}|$ , where $\mathbf{h}$ represents the altitude $h$ as a vector.

The area of triangle ABC is half of this, or $S=\frac{|AB\times AC|}{2}$ .

The area of triangle ABC can also be expressed in terms of dot products as follows:

$\frac{\sqrt{(\mathbf{AB}\cdot\mathbf{AB})(\mathbf{AC}\cdot\mathbf{AC})-(\mathbf{AB}\cdot\mathbf{AC})^2}}{2}=\frac{\sqrt{|\mathbf{AB}|^2|\mathbf{AC}|^2-(\mathbf{AB}\cdot\mathbf{AC})^2}}{2}$

### Using trigonometry

The height of a triangle can be found through an application of trigonometry. Using the labelling as in the image on the left, the altitude is $h=a\sin(\gamma)$ . Substituting this in the formula $S=\frac{bh}{2}$ derived above, the area of the triangle can be expressed as:

$S=\frac{ab\sin(\gamma)}{2}=\frac{bc\sin(\alpha)}{2}=\frac{ca\sin(\beta)}{2}$

Furthermore, since $\sin(\alpha)=\sin(\pi-\alpha)=\sin(\beta+\gamma)$ , and similarly for the other two angles:

$S=\frac{ab\sin(\alpha+\beta)}{2}=\frac{bc\sin(\beta+\gamma)}{2}=\frac{ca\sin (\gamma+\alpha)}{2}$

For a right triangle:

$A=\frac{\sin\left(\frac{360}{n}\right)}{4}s^2=\frac{s^2}{4\csc\left(\frac{360}{n}\right)}$

### Using coordinates

If vertex A is located at the origin $(0,0)$ of a Cartesian coordinate system and the coordinates of the other two vertices are given by $B=(x_B,y_B)$ and $C=(x_C,y_C)$ , then the area $S$ can be computed as ½ times the absolute value of the determinant

$S=\frac12\left|\det\begin{pmatrix}x_B&x_C\\y_B&y_C\end{pmatrix}\right|=\frac{\big|x_By_C-x_Cy_B\big|}{2}$

For three general vertices, the equation is:

\begin{align} S&=\frac12\left|\det\begin{pmatrix}x_A&x_B&x_C\\y_A&y_B&y_C\\1&1&1\end{pmatrix}\right|\\&=\frac{\big|x_Ay_C-x_Ay_B+x_By_A-x_By_C+x_Cy_B-x_Cy_A\big|}{2}\\&=\frac{\bigl|(x_C-x_A)(y_B-y_A)-(x_B-x_A)(y_C-y_A)\bigr|}{2} \end{align}

In 3 dimensions, the area of a general triangle $A=(x_A,y_A,z_A)\ ,\ B=(x_B,y_B,z_B)\ ,\ C=(x_C,y_C,z_C)$ is the Pythagorean sum of the areas of the respective projections on the three principal planes (i.e. $x=0,y=0,z=0$):

$S=\frac12\sqrt{\det\begin{pmatrix}x_A&x_B&x_C\\y_A&y_B&y_C\\1&1&1\end{pmatrix}^2+\det\begin{pmatrix}y_A&y_B&y_C\\z_A&z_B&z_C\\1&1&1\end{pmatrix}^2+\det\begin{pmatrix}z_A&z_B&z_C\\x_A&x_B&x_C\\1&1&1\end{pmatrix}^2}$

### Using Heron's formula

The shape of the triangle is determined by the lengths of the sides alone. Therefore the area $S$ also can be derived from the lengths of the sides. By Heron's formula:

\begin{align}S&=\sqrt{s(s-a)(s-b)(s-c)}\\S&=\frac{\sqrt{P(P-2a)(P-2b)(P-2c)}}{4}\end{align}

where $s=\frac{a+b+c}{2}$ is the semiperimeter, or half of the triangle's perimeter.

3 equivalent ways of writing Heron's formula are

\begin{align}S&=\frac{\sqrt{2\big(a^2b^2+c^2(a^2+b^2)\big)-a^4-b^4-c^4}}{4}\\ S&=\frac{\sqrt{(a^2+b^2+c^2)^2-2(a^4+b^4+c^4)}}{4}\\S&=\frac{\sqrt{2(a^2b^2+a^2c^2+b^2c^2)-a^4-b^4-c^4}}{4}\\S&=\frac{\sqrt{(a+b-c)(a-b+c)(b-a+c)(a+b+c)}}{4}\end{align}

## Computing the sides and angles

In general, there are various accepted methods of calculating the length of a side or the size of an angle. Whilst certain methods may be suited to calculating values of a right-angled triangle, others may be required in more complex situations.

### Trigonometric ratios in right triangles

In right triangles, the trigonometric ratios of sine, cosine and tangent can be used to find unknown angles and the lengths of unknown sides. The sides of the triangle are known as follows:

• The hypotenuse is the side opposite the right angle, or defined as the longest side of a right-angled triangle, in this case $h$ .
• The opposite side is the side opposite to the angle we are interested in, in this case $a$ .
• The adjacent side is the side that is in contact with the angle we are interested in and the right angle, hence its name. In this case the adjacent side is $b$ .

#### Sine, cosine and tangent

The sine of an angle is the ratio of the length of the opposite side to the length of the hypotenuse. In our case

$\sin(A)=\frac{\text{opposite}}{\text{hypotenuse}}=\frac{a}{h}$

Note that this ratio does not depend on the particular right triangle chosen, as long as it contains the angle $A$ , since all those triangles are similar.

The cosine of an angle is the ratio of the length of the adjacent side to the length of the hypotenuse. In our case

$\cos(A)=\frac{\text{adjacent}}{\text{hypotenuse}}=\frac{b}{h}$

The tangent of an angle is the ratio of the length of the opposite side to the length of the adjacent side. In our case

$\tan(A)=\frac{\text{opposite}}{\text{adjacent}}=\frac{a}{b}$

The acronym "SOHCAHTOA" is a useful mnemonic for these ratios.

There are many useful mnemonics that go with SOHCAHTOA, like "Some Old Hippie Caught A High Tripping On Acid"

Another useful mnemonic is "Some People Have Curly Brown Hair Turned Permanently Brown"

Here: S- sin , C- cos , T- tan , P- Perpendicular ( corresponds to Opposite ) , B- Base ( corresponds to Adjacent ) , H- Hypotenuse.

Eg: Some People Have ---- sin A = P/H

#### Inverse functions

The inverse trigonometric functions can be used to calculate the internal angles for a right angled triangle with the length of any two sides.

Arcsin can be used to calculate an angle from the length of the opposite side and the length of the hypotenuse.

$\theta=\arcsin\left(\frac{\text{opposite}}{\text{hypotenuse}}\right)$

Arccos can be used to calculate an angle from the length of the adjacent side and the length of the hypontenuse.

$\theta=\arccos\left(\frac{\text{adjacent}}{\text{hypotenuse}}\right)$

Arctan can be used to calculate an angle from the length of the opposite side and the length of the adjacent side.

$\theta=\arctan\left(\frac{\text{opposite}}{\text{adjacent}}\right)$

### The sine and cosine rules

The law of sines, or sine rule,[2] states that the ratio of the length of side $a$ to the sine of its corresponding angle $\alpha$ is equal to the ratio of the length of side $b$ to the sine of its corresponding angle $\beta$ .

$\frac{a}{\sin(\alpha)}=\frac{b}{\sin(\beta)}=\frac{c}{\sin(\gamma)}$

The law of cosines, or cosine rule, connects the length of an unknown side of a triangle to the length of the other sides and the angle opposite to the unknown side. As per the law:

For a triangle with length of sides $a,b,c$ and angles of $\alpha,\beta,\gamma$ respectively, given two known lengths of a triangle $a,b$ , and the angle between the two known sides $\gamma$ (or the angle opposite to the unknown side $c$), to calculate the third side $c$ , the following formula can be used:

\begin{align}c^2&=a^2+b^2-2ab\cos(\gamma)\\b^2&=a^2+c^2-2ac\cos(\beta)\\a^2&=b^2+c^2-2bc\cos(\alpha)\end{align}

For a right triangle:

Adjacent: $cos\left(\frac{180}{n}\right),A=b^2+c^2-a^2,B=a^2+c^2-b^2,C=a^2+b^2-c^2$
Opposite: $sin\left(\frac{180}{n}\right),\sqrt{(a^2+b^2+c^2)^2-2(a^4+b^4+c^4)}$
Hypotenuse: 1, $A=2bc,B=2ac,C=2ab$

### Trigonometric functions of half angles in a triangle

\begin{align} \cos&=\sqrt{\frac{P^2}{4bc}-\frac{aP}{2bc}},\sqrt{\frac{P^2}{4ac}-\frac{bP}{2ac}},\sqrt{\frac{P^2}{4ab}-\frac{cP}{2ab}}\\ \sin&=\sqrt{\frac{(P-2c)(P-2b)}{2bc}},\sqrt{\frac{(P-2c)(P-2a)}{2ac}},\sqrt{\frac{(P-2b)(P-2a)}{2ab}}\\ \tan&=\sqrt{\frac{4(P-2c)(P-2b)}{4P^2-8Pa}}\\ \sec&=\sqrt{\frac{4bc}{P^2-2Pa}}\\ \csc&=\sqrt{\frac{4bc}{(P-2c)(P-2b)}}\\ \cot&=\sqrt{\frac{4P^2-8Pa}{4(P-2c)(P-2b)}} \end{align}

## Non-planar triangles

A non-planar triangle is a triangle which is not contained in a (flat) plane. Examples of non-planar triangles in noneuclidean geometries are spherical triangles in spherical geometry and hyperbolic triangles in hyperbolic geometry.

While all regular, planar (two dimensional) triangles contain angles that add up to 180°, there are cases in which the angles of a triangle can be greater than or less than 180°. In curved figures, a triangle on a negatively curved figure ("saddle") will have its angles add up to less than 180° while a triangle on a positively curved figure ("sphere") will have its angles add up to more than 180°. Thus, if one were to draw a giant triangle on the surface of the Earth, one would find that the sum of its angles were greater than 180°. In this circumstance, one can even make each of the triangle in question's angles measure 90°, adding up to a total of 270°.