L1.21 - One Hundred and Eighty

Learning Intentions and Success Criteria

We are learning to:

Prove (in writing) that the sum of the measures of the angles in a triangle is 180 degrees.

We are successful when we can:

Prove the angles in a triangle sum to 180 degrees.

1. Your teacher will assign you one of Tyler’s statements to think about. Is the statement true? Explain your reasoning.

2. In what circumstances are corresponding angles congruent? Be prepared to share your reasoning.

21.2: Triangle Angle Sum One Way

1. Use technology to create a triangle. Use the Text tool to label the 3 interior angles as a, b, and c. (Done in applet) 2. Mark the midpoints of 2 of the sides. 3. Extend the side of the triangle without the midpoint in both directions to make a line. 4. Use what you know about rotations to create a line parallel to the line you made that goes through the opposite vertex. 5. What is the value of a + b + c? Explain your reasoning.

21.3: Triangle Angle Sum Another Way

1. Translate triangle ABC along the directed line segment from B to C to make triangle A’B’C’. Label the measures of the angles in triangle A’B’C’. 2. Translate triangle A’B’C’ along the directed line segment from A’ to C to make triangle A”B”C”. Label the measures of the angles in triangle A”B”C”. 3. Label the measures of the angles that meet at point C. Explain your reasoning. 4. What is the value of a + b + c? Explain your reasoning.