Triangles

Classify triangles

Triangles can be classified by their angles:
 Acute Right Obtuse All interior angles are acute (<90º) One angle is a right angle (90º) One angle is obtuse (>90º)

That is, you can classify triangles based on the measures of their angles.

• In an acute triangle, all angles measure less than 90º.
• In a right triangle, one angle measures exactly 90º.
• In an obtuse triangle, one angle measures more than 90º.

You can also classify triangles based on the lengths of their sides.

 Equilateral Isosceles Scalene All three sides have equal lengths Exactly two equal sides All sides have different lengths

That is, you can classify triangles based on the lengths of their sides.

• In an equilateral triangle, all three sides are the same length.
An equilateral triangle has 3 equal angles.
• In an isosceles triangle, two sides are the same length.
An isosceles triangle has 2 equal angles.
• In a scalene triangle, the three sides have different lengths.
A scalene triangle has 3 different angles.

 What kind of triangle is this?

There is one right angle. The other two angles are acute. Therefore, it is a right triangle.

 What kind of triangle is this?

One of the angle measures is more than 90º. Therefore, it is an obtuse triangle.

 What kind of triangle is this?

The three sides have the same length. Therefore, it is an equilateral triangle.

Is this triangle scalene?

yes
no