triangle it is a geometric figure formed by three straight lines that meet two by two and do not pass through the same point, forming three sides and three angles.
To calculate the perimeter of a triangle just add up the measurement from all sides, the sum of internal angles it is always 180º.
Observing the triangle, we can identify some of its elements:
♦ A, B and C are the vertices.
♦ The sides of the triangles are symbolized by the meeting of the vertices (meeting points): 
, 
, 
straight segments.
♦ Angles have two ways to represent them: in the case of a triangle it has 3 sides, therefore, 3 angles: Â, 
,? Or the 
C, B? A, BAC.
Types of triangles
♦ The triangle can be classified according to the measure of its side.
Scalene triangle: All sides and angles are different.
Isosceles triangles: two equal sides and the opposite angles to these equal sides. 
Equilateral triangle: All sides and angles equal. We conclude that your angles will be 60°.
♦ The triangle can be classified according to its internal angles.
Rectangle triangle: has an angle that measures 90º.
Obtusangle: has an angle greater than 90°.
Acute-angle: Has all angles less than 90°.
Condition of existence of a triangle
To build a triangle we cannot use any measure, it has to follow the existence condition:
To build a triangle it is necessary that the measure of either side is smaller than the sum of the measures of the other two and greater than the absolute value of the difference between these measures.
| b - c | < a < b + c
| a - c | < b < a + c
| a - b | < c < a + b
Example:
14 – 8 < 10 < 14 + 10
14 – 10 < 8 < 14 + 10
10 – 8 < 14 < 10 + 8
by Danielle de Miranda
Graduated in Mathematics
