Trigonometry/Triangle Geometry
Trilaterometry or Triangle Geometry refers to the aspect of plane geometry which deals with triangles, which are the the most basic polygon with only 3 (minimum possible) sides. Its importance can be justified by the fact that all polygons can cut into triangles with all its vertices being vertices of original polygon.
Theorems[edit  edit source]
Area of a Triangle[edit  edit source]
Calculating the area of a triangle is an elementary problem encountered often in many different situations. The best known and simplest formula is:
where is area, is the length of the base of the triangle, and 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.
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' with the 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:
where s = ½ (a + b + c) is the semiperimeter, or half of the triangle's perimeter.
Three equivalent ways of writing Heron's formula are
Heron's formula is a special case of Brahmagupta's Theorem.
Intercept Theorem[edit  edit source]
The intercept theorem is an important theorem in elementary geometry about the ratios of various line segments, that are created if 2 intersecting lines are intercepted by a pair of parallels. It is equivalent to the theorem about ratios in similar triangles. Traditionally it is attributed to Greek mathematician Thales, which is the reason why it is named theorem of Thales in some languages.
Theorem and Proof:
Due to heights of equal length ( ) we have and therefore . This yields Plugging in the actual formula for triangle areas () transforms that into Cancelling the common factors results in:
Now use (b) to replace and in (a):

Pythagorean Theorem[edit  edit source]
In mathematics, the Pythagorean theorem (American English) or Pythagoras' theorem (British English) is a relation in Euclidean geometry among the three sides of a right triangle. The theorem is usually written as an equation:
where c represents the length of the hypotenuse, and a and b represent the lengths of the other two sides. In words:
The square of the hypotenuse of a right triangle is equal to the sum of the squares on the other two sides.
The Pythagorean theorem is named after the Greek mathematician Pythagoras, who by tradition is credited with its discovery and proof, although it is often argued that knowledge of the theory predates him. (There is much evidence that both Babylonians and Indian understood the principle).
Proof 1:
Like most of the proofs of the Pythagorean theorem, this one is based on the proportionality of the sides of two similar triangles.
Let ABC represent a right triangle, with the right angle located at C, as shown on the figure. We draw the altitude from point C, and call H its intersection with the side AB. The new triangle ACH is similar to our triangle ABC, because they both have a right angle (by definition of the altitude), and they share the angle at A, meaning that the third angle will be the same in both triangles as well. By a similar reasoning, the triangle CBH is also similar to ABC. The similarities lead to the two ratios..:
As
so
These can be written as
Summing these two equalities, we obtain
In other words, the Pythagorean theorem:
Proof 2 (as given in Baudhayana SulbaSutra):
Looking at the illustration which is a large square with identical right triangles in its corners, the area of each of these four triangles is given by an angle corresponding with the side of length C.
The Aside angle and Bside angle of each of these triangles are complementary angles, so each of the angles of the blue area in the middle is a right angle, making this area a square with side length C. The area of this square is C^{2}. Thus the area of everything together is given by:
However, as the large square has sides of length A + B, we can also calculate its area as (A + B)^{2}, which expands to A^{2} + 2AB + B^{2}.
 (Distribution of the 4)
 (Subtraction of 2AB)
Angle Bisector Theorem[edit  edit source]
In geometry, the angle bisector theorem relates the length of the side opposite one angle of a triangle to the lengths of the other two sides of the triangle.
Consider a triangle ABC. Let the angle bisector of angle A intersect side BC at a point D. The angle bisector theorem states that the ratio of the length of the line segment BD to the length of segment DC is equal to the ratio of the length of side AB to the length of side AC
The generalized angle bisector theorem states that if D lies on BC, then
This reduces to the previous version if AD is the bisector of BAC.
Proof of generalization:
Let B_{1} be the base of altitude in the triangle ABD through B and let C_{1} be the base of altitude in the triangle ACD through C. Then,
It is also true that both the angles DB_{1}B and DC_{1}C are right, while the angles B_{1}DB and C_{1}DC are congruent if D lies on the segment BC and they are identical otherwise, so the triangles DB_{1}B and DC_{1}C are similar (AAA), which implies that
Q.E.D.
Apollonius' Theorem[edit  edit source]
In elementary geometry, Apollonius' theorem is a theorem relating several elements in a triangle.
It states that given a triangle ABC, if D is any point on BC such that it divides BC in the ratio n:m (or ), then
Special cases of the theorem:
 When , that is, AD is the median falling on BC, the theorem reduces to
 When in addition AB = AC, that is, the triangle is isosceles, the theorem reduces to the Pythagorean theorem,
In simpler words, in any triangle , if is a median, then
Ceva's Theorem[edit  edit source]
In geometry, Ceva's theorem is a theorem about triangles. Given a triangle ABC, let the lines AO, BO and CO be drawn from the vertices to a common point O (not on one of the sides of ABC), to meet opposite sides at D, E and F respectively. (The segments AD, BE, and CF are known as cevians.) Then, using the lengths of segments,
Examples[edit  edit source]
Resources[edit  edit source]
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Practice Questions[edit  edit source]
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