Applying Triangle Angle Theorems

Print Lesson


SWBAT Apply triangle angle theorems to find the measures of missing angles

Big Idea

Students use their knowledge of theorems from this unit to find the measures of angles in a complex diagram involving triangles.

Do Now

7 minutes

In the Do Now, students are asked to identify numbered angles using letters. Students have difficulty identifying angles using letters. This activity allows students to practice correctly identifying angles and informs me of the students who need more practice or clarification. After about 4 minutes, we go over the angles and discuss the different ways the angles could be named.


10 minutes

The Mini-Lesson begins with a review of the terms vertical angles, alternate interior angles, and linear pair. Students identify angles of the diagram from the Do Now. Since there are multiple answers for each question, I call on multiple students to provide answers until we provide a relatively complete description of the angles in the diagram. 

We then go over some angle theorems using angles from the diagram. I plan to have my students answer questions based on their knowledge of postulates and theorems. These questions will help them complete the activity later in the lesson.


23 minutes

Applying Triangle Angle Theorems Activity asks students to interpret and use given information about a diagram. Using this information and their knowledge of theorems and postulates, students work together to figure out the measures of each of the angles in the diagram. After they find the measures of the angles, they apply their knowledge to answer questions about parts of the diagram.

Standard G.CO.10 states, "Prove theorems about triangles." Although this activity does not explicitly have students prove theorems, they are using theorems they proved in previous lessons.

Some students need additional guidance in order to answer these questions. I work with small groups to discuss the theorems needed. If students are still having difficulty, I provide them with an alternative activity to work on. This activity provides more scaffolding for these students.


5 minutes

To end the lesson, we go over the measures of the angles in the diagram. I call on students to give the measures of angles and explain how they found the measure (MP3). We then discuss answers to the questions about the angles.