Types of Triangles, Angles, and Sides

Types of Triangles

​

Classifying triangles by their sides and lengths

• Triangles have special names based on their side lengths and angle measures.

  
  

  
  

• When you classify triangles by their side lengths, you use the words equilateral, isosceles, and scalene.

Equilateral Triangle

• An equilateral triangle has three equal sides.

• This is an example of a equilateral triangle.

Multiple Choice

Which of these triangles is equilateral?

Isosceles triangle

• An isosceles triangle has two equal sides.

• This is an example of an isosceles triangle.

Multiple Choice

Which of these triangles is isosceles?

Scalene Triangle

• A scalene triangle has no equal sides.

• This is an example of scalene triangle.

Multiple Choice

Which of these triangles is scalene?

Classifying triangles by angle measure

• When you classify triangles by their angle measures, you use the words acute, right, and obtuse.

  
  

• Let's find out what those words mean!

Acute Triangle

• An acute triangle has three acute angles.

• Acute angles measure less than 90°

• This is an example of an acute triangle.

Multiple Choice

Which of these is an acute triangle?

Right Triangle

• A right triangle has one angle.

• Right angles measure exactly 90°.

• This is an example of a right triangle.

• Notice that all of the angles add up to be 180 degrees.

• The angles in a triangle always add up to 180 degrees

Multiple Choice

Which of these is a right triangle?

Obtuse triangle

• An obtuse triangle has one obtuse angle.

• Obtuse angles measure more than 90°.

• This is an example of an obtuse triangle.

Multiple Choice

Which of these triangles is obtuse?

Classifying triangles two ways

• This is a right isosceles triangle.

• It has a right angle and two equal sides.