Lesson 9 Lines Angles Special Triangles WarmUp Two

Lesson 9 Lines & Angles Special Triangles

Warm-Up Two angles in a triangle are listed. Find the measure of the third angle. 1. 90º, 30º, ____ 2. 129º, 42º, ____ 3. 22º, 55º, ____ 4. 22. 5º, 46. 3º, ____

Special Triangles Target: Find measures of angles in isosceles and equilateral triangles.

Vocabulary � Equiangular: All angles have the same measure.

Equilateral and Isosceles Triangle Angle Properties � Equilateral � Isosceles Triangle: Each angle in an equilateral triangle is 60°. Triangle : The angles which are across from the congruent sides will be equal in measure.

Example 1 ΔMNP is an equilateral triangle. The measure of M is (2 x + 6)°. Find the value of x. Each angle in an equilateral triangle is 60°. Set the angle equal to 60°. 2 x + 6 = 60 Subtract 6 from each side. – 6 2 x = 54 Divide by 2 on each side. 2 2 x = 27 The value of x is 27.

Example 2 Find the value of x. � The triangle is isosceles. The angles that are across from the congruence marks must be equal. So, 112 + x = 180 � Combine like terms. 112 + 2 x = 180 � Subtract 112 from each side. – 112 2 x = 68 � Divide by 2 on each side. 2 2 x = 34 � The value of x is 34.

Exit Problems Find the value of x.

Communication Prompt Do you think there any special angle relationships in a scalene triangle? Explain your answer.