This document discusses the angles of triangles. It explains that the three angles of any triangle will sum to 180 degrees. If two angles of a triangle are known, the third unknown angle can be found by subtracting the two known angles from 180 degrees. Examples are provided to demonstrate finding the measure of the unknown third angle of a triangle given two other angle measures.

1. Geometry Angles of a Triangle Objective: Outcome E3 – Make and apply generalizations about the sum of the angles in triangles and quadrilaterals.

2. Sum of the angles of a triangle a c b

3. Angles of a Triangle a c b Draw any triangle on a piece of paper. Mark the corners a, b and c. Tear the triangle into three corners.

4. Angles of a Triangle Draw any triangle on a piece of paper. Mark the corners a, b and c. Tear the triangle into three corners. a c b

5. Angles of a Triangle Arrange the three pieces so the corners are adjoining. What do you notice? a c b

6. Angles of a Triangle What do you notice? What kind of angle to they form? What is the measure of the angle? a c b

7. Angles of a Triangle The sum of the angles of any triangle equals 180 °. a c b a + b + c = 180 °

8. Finding Angles of a Triangle If we know two angles of a triangle, we can find the third angle by subtracting the first two from 180 °. 80 ° 40 ° ? 180 80 40 ? ? =

9. Finding Angles of a Triangle If we know two angles of a triangle, we can find the third angle by subtracting the first two from 180 °. 80 ° 40 ° ? 180 80 40 60 ? = 60 °

10. Finding Angles of a Triangle Find the measure of the unknown angle. 100 ° 35 ° ?

11. Finding Angles of a Triangle Find the measure of the unknown angle. 50° 75 ° ?