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A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three
sides or edges which are line segments. A triangle with vertices A, B, and C is denoted .

Types of triangle:-Euler diagram of types of triangles, using the definition that isosceles triangles have
at least 2 equal sides, i.e. equilateral triangles are isosceles.

By relative lengths of sides:-Triangles can be classified according to the relative lengths of their
sides:In an equilateral triangle all sides have the same length. An equilateral triangle is also a regular
polygon with all angles measuring 60°.In an isosceles triangle, two sides are equal in length.An
isosceles triangle also has two angles of the same measure; namely, the angles opposite to the two sides
of the same length;

This fact is the content of the Isosceles triangle theorem. Some mathematicians define an isosceles
triangle to have exactly two equal sides, whereas others define an isosceles triangle as one with at least
two equal sides.[3] The latter definition would make all equilateral triangles isosceles triangles. The 45–
45–90 Right Triangle, which appears in the Tetrakis square tiling, is isosceles.ateral.

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In diagrams representing triangles (and other geometric figures), "tick" marks along the sides are used
to denote sides of equal lengths – the equilateral triangle has tick marks on all 3 sides, the isosceles on 2
sides. The scalene has single, double, and triple tick marks, indicating that no sides are equal. Similarly,
arcs on the inside of the vertices are used to indicate equal angles. The equilateral triangle indicates all
3 angles are equal; the isosceles shows 2 identical angles. The scalene indicates by 1, 2, and 3 arcs that
no angles are equal.

By internal angles

Triangles can also be classified according to their internal angles, measured here in degrees.A right
triangle (or right-angled triangle, formerly called a rectangled triangle) has one of its interior angles
measuring 90° (a right angle). The side opposite to the right angle is the hypotenuse; it is the longest
side of the right triangle. The other two sides are called the legs or catheti[5] (singular: cathetus) of the

Right triangles obey the Pythagorean theorem: the sum of the squares of the lengths of the two legs is
equal to the square of the length of the hypotenuse: a2 + b2 = c2, where a and b are the lengths of the
legs and c is the length of the hypotenuse. Special right triangles are right triangles with additional
properties that make calculations involving them easier. One of the two most famous is the 3–4–5 right
triangle, where 32 + 42 = 52. In this situation, 3, 4, and 5 are a Pythagorean Triple. The other one is an
isosceles triangle that has 2 angles that each measure 45 degrees.

Triangles that do not have an angle that measures 90° are called oblique triangles.A triangle that has all
interior angles measuring less than 90° is an acute triangle or acute-angled triangle.A triangle that has
one angle that measures more than 90° is an obtuse triangle or obtuse-angled triangle.

A "triangle" with an interior angle of 180° (and collinear vertices) is degenerate.A triangle that has two
angles with the same measure also has two sides with the same length, and therefore it is an isosceles
triangle. It follows that in a triangle where all angles have the same measure, all three sides have the
same length, and such a triangle is therefore equil

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