Triangles I can describe and classify triangles by their angles. Modified by L. Standifer 1/2010

Big Idea: Triangles can be classified by their angles. What are the different types of triangles? How is each described? Essential Question

Think-Pair-Share Let’s review what you know about angles. What is a right angle? exactly 90°, Square Corner What is an obtuse angle? 91°-179°, Bigger than a 90° right angle What is an acute angle? Smaller than 90°, “aww” cute

Two ways to classify triangles... You may describe and classify triangles by their inside angles or by the length of their sides. Today, we will classify triangles by their angles.

describe and classify triangles? How do angles help us describe and classify triangles? In order to describe and classify triangles by their angles, we need to use the same descriptions we used to identify angles. As we learn about classifying angles, you will notice that we must label each of the three angles inside a triangle.

Right Triangle If it has a square corner - it’s a right triangle! A right triangle has exactly ONE right angle. If it has a square corner - it’s a right triangle!

Right Triangle Steps to classify triangles: 1. Label all of the corners. 2. Count how many angles are right, acute, or obtuse. 3. Classify the type by how many types of angles a triangle has. Acute This one has only one right angle and two acute angles, so it is a RIGHT TRIANGLE. RIGHT Acute

One more helpful step... You may use a paper with a square corner to check a right triangle! Acute Line up the corner of the right angle triangle and the corner of the paper to see if it is exactly 90°. Acute Right

Yes, this is also a right triangle! Label the angles to classify these triangles. Use a corner of paper to check. Do all of the triangles have 1 right angle and 2 acute angles? Yes, this is also a right triangle!

...and more right triangles!

Obtuse Triangle obtuse Has one obtuse angle - An obtuse triangle has exactly ONE obtuse angle. If it has an obtuse angle - it’s an obtuse triangle! obtuse

Obtuse Triangle Remember to use the classifying triangle steps! 1. Label all of the corners. 2. Count how many angles are right, acute, or obtuse. 3. Classify the type by how many types of angles a triangle has. acute If it has an obtuse angle - it’s an obtuse triangle! obtuse acute

Obtuse triangles... Sometimes an angle is hard to classify. You can use the paper trick to make sure an angle is bigger than a 90° angle.

Acute Triangle (or AAA triangle) acute acute acute Has three acute angles - ALL the angles are acute. If you check all three angles and they are smaller than 90°, it’s acute! (or AAA triangle) acute acute

Are these all acute triangles? Yes, all of these have three acute angles, so they are all acute triangles!

Take a look at these triangles - what are they? 1. 3. 2. 6. 4. 5. Take out your slate and number it - then tell me what it is!

How did you do? 1. Acute Obtuse 3. Right 2. Right 6. 4. 5. Acute

Showdown - Using your slates... Tell me what makes a right triangle? What makes an obtuse triangle? What is an acute triangle? Answer each question - then show and share with your group. Compare answers and discuss any differences.