Unit 1 - Lesson 16 - Grade 8: Illustrative Mathmatics

Look at angles EAB, BAC, and CAD. Without measuring, write what you think is the sum of the measures of these angles. Explain or show your reasoning.

Is the measure of angle EAB equal to the measure of any angle in triangle ABC? If so, which one? If not, how do you know?

Is the measure of angle CAD equal to the measure of any angle in triangle ABC? If so, which one? If not, how do you know?

What is Explain how you know.

Use your answer to explain why a + b + c = 180.

Explain why your argument will work for any triangle: that is, explain why the sum of the angle measures in any triangle is 180°.

Given that angle BAC measures 53 degrees, find as many other angle measures as you can.

In an equilateral triangle, all side lengths are equal and all angle measures are equal. Sketch an equilateral triangle. What are the measures of its angles?