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In [[Euclidean geometry]], each side of a triangle is perfectly straight, and the sum of the internal angles of a triangle is always 180º.

In [[Euclidean geometry]], each side of a triangle is perfectly straight, and the sum of the internal angles of a triangle is always 180º.

==Types of triangles==

==Types of triangles==

An '''obtuse triangle''' has one angle that measures more than 90^o.

An '''obtuse triangle''' has one angle that measures more than 90^o.

==Naming conventions==

{| class="wikitable"

Usually, the vertices of a triangle are counter-clockwise denoted by big Latin letters, while small Latin letters are used for the sites: Every site will have the same letter as the opposite vertex.  

The angles are denoted by Greek letters, if possible, the letter of an angle will correspond to the letter of the adjacent vertex.

|<!--col1-->[[Image:Right-triangle.png|150px]]

Often, the sides will be referenced by the adjacent vertices: in the triangle on top of this page, we have <math>a = \overline{BC} = \overline{CB}, b = \overline{CA} = \overline{CA}, c = \overline{BA} = \overline{AB} </math>.

|<!--col2-->[[Image:Isoscles-triangle.png|150px]]

Similarly, the angles are denoted as <math>\alpha = \angle CAB, \beta = \angle ABC, \gamma = \angle BCA</math>.

|<!--col3-->[[Image:Equilateral-triangle.png|150px]]

|<!--col4-->[[Image:Obtuse-triangle.jpg|150px]]

|<!--col1--><math>\angle CBA =90^\circ = \pi/2</math>

|<!--col2--><math>\angle BCA = \angle CAB </math>

|<!--col3--><math>\angle ABC = \angle BCA = \angle CAB</math>

|<!--col4--><math>\angle ACB > 90^\circ</math>

 

==Congruence of triangles==

==Congruence of triangles==