Common Core Math Vocabulary

Triangle - Definition with Examples

What is a Triangle?

In geometry, a triangle is a closed, two-dimensional shape with three straight sides. A triangle is also a polygon

Triangle math geometry




We can find the shape of a triangle in a flag, the musical instrument triangle, and a roadside signboard. 

Triangle shape real life things


Properties of a Triangle

Triangle properties angle sides

  • A triangle has three sides, three vertices, and three angles.
  • The sum of the three interior angles of a triangle is always 180°.
  • The sum of the length of two sides of a triangle is always greater than the length of the third side.
  • A triangle with vertices P, Q, and R is denoted as △PQR.
  • The area of a triangle is equal to half of the product of its base and height.


Different Types of Triangles

To classify triangles according to their angles, we measure each of its interior angles. Triangles can be classified by angles, as:

  • Acute Triangle
  • Right Triangle
  • Obtuse Triangle

Types of triangles by angles

An acute triangle has all interior angles acute (less than 90°), a right triangle has one right angle (equal to 90°) and an obtuse triangle has one obtuse angle (greater than 90°).


To classify the triangles according to their sides, we measure the length of each of its sides. Triangles can be classified by their sides, as:

  • Equilateral Triangle
  • Isosceles Triangle
  • Scalene Triangle

Types of triangles by sides

To classify triangles according to both angles and sides, we measure the interior angles and length of the sides of the triangle. Few examples of triangles classified on the basis of both angles and sides are:

  • Acute Equilateral Triangle
  • Right Isosceles Triangle
  • Obtuse Scalene Triangle

Types of triangles by both sides and angles

  Fun Facts
  • A triangle can always be split into two right triangles, irrespective of its orientation.
  • Triangles are polygons with the least number of sides.


let's sing Let's sing!

Jenna loves triangles!
Tiny triangles on her blouse,
The red roof of her dollhouse,
A piece of cheese for her mouse.


let's do it Let's do it!

Instead of handing out math worksheets to your children, ask your child to observe/spot the things that are triangular in shape. 
You can also give them toothpicks or straws and ask them to form all the different types of triangles using them.


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