Lesson 9
1. Lesson 9
1.5. Explore 4
Module 2: Logic and Geometry
In the preceding Share activity, you and your partner were asked to make a conjecture about the relationship between an exterior angle and its two non-adjacent interior angles. Zahia and her partner made the following conjecture.
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Use the Exterior Angles of Triangles applet to see if Zahia’s conjecture holds true for other triangles. Drag the orange dots on any vertex to reshape the triangle.
The Exterior Angles of Triangles applet illustrates the relationship between the exterior and interior angles of a triangle. Read “Example 2: Using reasoning to determine the relationship between the exterior and interior angles of a triangle” on page 88 of your textbook to see a proof of this relationship. As you read through Joanna’s proof, consider how she uses angle properties you have learned in previous lessons to prove the relationship between an exterior angle of a triangle and the two non-adjacent interior angles.
Self-Check 2