A triangle is composed of three line segments. The line segments
intersect in their endpoints. To name a triangle we often use its
vertices (the name of the endpoints). The triangle below is named
A triangle has three angles. The sum of the measures of the
angles is always 180° in a triangle.
We have different types of triangles. A triangle is classified
by its angles and by the number of congruent sides.
A triangle that has three acute angels is called an acute
A triangle that has one right angle is called a right
A triangle that has one obtuse angle is called an obtuse
When a triangle has three congruent sides, we call the triangle
an equilateral triangle. We mark the congruent sides by a slash
mark. The angles in an equilateral triangle are always 60°.
When a triangle has two congruent sides it is called an
isosceles triangle. The angles opposite to the two sides of the
same length are congruent.
A triangle without any congruent sides or angles is called a
When two triangles are congruent it means that they have the
same size and shape. This means that they have the same angles. The
red slash marks show us which sides and angles that are congruent
Congruency is shown by this symbol
Triangles that have congruent angles but not the same size are
called similar. Similar triangles have sides that are proportional.
Similarity is shown by this symbol
Find x in the similar triangles.
We know that since the triangles are similar the sides are
proportional which means that
Video lesson: Find out whether the triangles
are right, isosceles, acute, scalene, obtuse or equilateral