Mathematics
Grade 3
15 min
Acute, obtuse, and right triangles
Acute, obtuse, and right triangles
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Identify the three types of angles: acute, right, and obtuse.
Define a triangle as a shape with three sides and three angles.
Classify a triangle as a right triangle by identifying its right angle.
Classify a triangle as an acute triangle by identifying that all three of its angles are acute.
Classify a triangle as an obtuse triangle by identifying its one obtuse angle.
Sort a group of mixed triangles into acute, obtuse, and right categories.
Draw an example of each type of triangle.
Have you ever noticed that a slice of pizza is a triangle? 🍕 What kind of special name can we give that triangle shape?
Today, we are going to be triangle detectives! We will learn how to look at the corners, or angles, of any triangle and give it a special name: acute, o...
2
Key Concepts & Vocabulary
TermDefinitionExample
AngleThe space between two lines that meet at a point. We can think of it as a corner.The corner of a square is an angle.
Right AngleA special angle that looks like the corner of a square or a piece of paper. It makes a perfect 'L' shape.The corners of your math book are all right angles.
Acute AngleAn angle that is smaller or 'skinnier' than a right angle. 'Acute' sounds like 'a-cute' little angle!The pointy tip of a brand new crayon is an acute angle.
Obtuse AngleAn angle that is bigger or 'wider' than a right angle. It's like a wide open mouth.The angle a fan makes when it opens up wide is an obtuse angle.
TriangleA flat shape with 3 straight sides and 3 angles.A yield sign on the road is a triangle.
3
Core Formulas
Right Triangle Rule
1 Right Angle + 2 Acute Angles = Right Triangle
If you can find one perfect 'L' shaped corner (a right angle) in a triangle, it's a right triangle. The other two angles will always be acute.
Obtuse Triangle Rule
1 Obtuse Angle + 2 Acute Angles = Obtuse Triangle
If a triangle has one big, wide angle (an obtuse angle), it's an obtuse triangle. The other two angles will always be acute.
Acute Triangle Rule
3 Acute Angles = Acute Triangle
If all three angles in a triangle are small and pointy (acute), it's an acute triangle. This is the only type where all three angles are the same kind.
4 more steps in this tutorial
Sign up free to access the complete tutorial with worked examples and practice.
Sign Up Free to ContinueSample Practice Questions
Challenging
Imagine a perfect square piece of paper. If you fold it exactly in half along the diagonal (from corner to corner), what is the best description of the triangle shape you create?
A.An acute triangle with three equal sides.
B.An obtuse triangle.
C.small square.
D.right triangle with two equal sides.
Challenging
A ship's sail is a large triangle. The mast (the tall pole) and the boom (the bottom pole) make a perfect corner. What two types of triangles could the sail be?
A.right triangle or an obtuse triangle
B.An acute triangle or an obtuse triangle
C.Only a right triangle
D.An acute triangle or a right triangle
Challenging
Imagine an analog clock. At 3:00, the hour hand and the minute hand form a right angle with the center of the clock. If you draw a line connecting the tip of the hour hand to the tip of the minute hand, what kind of triangle have you formed?
A.An acute triangle
B.right triangle
C.An obtuse triangle
D.It depends on the size of the clock.
Want to practice and check your answers?
Sign up to access all questions with instant feedback, explanations, and progress tracking.
Start Practicing Free