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The teachers  In a triangle there are two kinds of angles: The interior angles are formed by two sides.

The exterior angles are formed on one side and its extension.

## Properties of the Angles of a Triangle

1. The sum of the interior angles of a triangle equals 180°.

A + B + C = 180º

2.  The value of an exterior angle of a triangle equals the sum of the two nonadjacent interior angles.

α = B + C

3.   The interior and exterior angles of a triangle at a particular vertex are supplementary, that is to say, they add up 180º.

α = 180º - A

## Acute Triangle An acute triangle has three acute angles.

## Right Triangle A right triangle has a right angle.

The longest side of the triangle is called the hypotenuse.

The shorter sides of the triangle are the legs or catheti (singular: cathetus).

## Obtuse Triangle An obtuse triangle has an obtuse angle.

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