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

"Solution of triangles" is the main trigonometric problem: to find missing characteristics of a triangle (three angles, the lengths of the three sides etc.) when at least three of these characteristics are given. The triangle can be located on a plane or on a sphere. This problem often occurs in various trigonometric applications, such as geodesy, astronomy, construction, navigation etc.

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(F.J. n.d., p. 206; Borchardt and Perrott 1930) and

From the above angle sum formula we can also see that the Earth's surface is locally flat: If we draw an arbitrarily small triangle in the neighborhood of one point on the Earth's surface, the fraction f of the Earth's surface which is enclosed by the triangle will be arbitrarily close to zero. In this case the angle sum formula simplifies to 180°, which we know is what Euclidean geometry tells us for triangles on a flat surface.

If one reflects a median in 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.

A man who specializes in debunking paranormal occurrences checks into the fabled room 1408 in the Dolphin Hotel. Soon after settling in, he confronts genuine terror.

A woman who lives in a darkened old house with her two photosensitive children becomes convinced that her family home is haunted.

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

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

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Any triangle can be positioned such that its shadow under an orthogonal projection is equilateral.

The altitude from, for example, the side of length a is

Three formulas have the same structure as Heron's formula but are expressed in terms of different variables. First, denoting the medians from sides a, b, and c respectively as ma, mb, and mc and their semi-sum (ma + mb + mc)/2 as σ, we have[13]

Thales' theorem implies that if the circumcenter is located on one side of the triangle, then the opposite angle is a right one. 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.

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Morley's trisector theorem states that in any triangle, the three points of intersection of the adjacent angle trisectors form an equilateral triangle, called the Morley triangle.

Three other equivalent ways of writing Heron's formula are

The formulas in this section are true for all Euclidean triangles.

The definition of the semiperimeter leads to the definitions

The law of tangents or tangent rule, can be used to find a side or an angle when you know two sides and an angle or two angles and a side. It states that:[9]

(the SAS theorem). Finally, if all three sides are specified, a unique triangle is determined with area given by Heron's formula or by

Euler's theorem states that the distance d between the circumcenter and the incenter is given by[25]:p.85

both again holding if and only if the triangle is equilateral.

As discussed above, every triangle has a unique inscribed circle (incircle) that is interior to the triangle and tangent to all three sides.

Specifically, on a sphere the sum of the angles of a triangle is

The base can be any side, Just be sure the "height" is measured at right angles to the "base":

Any line through a triangle that splits both the triangle's area and its perimeter in half goes through the triangle's incenter. There can be one, two, or three of these for any given triangle.

There are three special names given to triangles that tell how many sides (or angles) are equal. There can be 3, 2 or no equal sides/angles:

Every triangle has a unique Steiner inellipse which is interior to the triangle and tangent at the midpoints of the sides. Marden's theorem shows how to find the foci of this ellipse.[30] This ellipse has the greatest area of any ellipse tangent to all three sides of the triangle.

Other upper bounds on the area T are given by[23]:p.290

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The area T of any triangle with perimeter p satisfies

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

Triangle

There are infinitely many lines that bisect the area of a triangle.[24] Three of them are the medians, which are the only area bisectors that go through the centroid. Three other area bisectors are parallel to the triangle's sides.

for semiperimeter s, where the bisector length is measured from the vertex to where it meets the opposite side.

Three positive angles α, β, and γ, each of them less than 180°, are the angles of a triangle if and only if any one of the following conditions holds:

Imagine you "doubled" the triangle (flip it around one of the upper edges) to make a square-like shape (a parallelogram) which can be changed to a simple rectangle:

Further, every triangle has a unique Steiner circumellipse, which passes through the triangle's vertices and has its center at the triangle's centroid. Of all ellipses going through the triangle's vertices, it has the smallest area.

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