Lesson 16Parallel Lines and the Angles in a Triangle
Learning Goal
Let’s see why the angles in a triangle add to 180 degrees.
Learning Targets
I can explain using pictures why the sum of the angles in any triangle is 180 degrees.
Lesson Terms
- alternate interior angles
- straight angle
- transversal
Warm Up: True or False: Computational Relationships
Problem 1
Is each equation true or false?
Activity 1: Angle Plus Two
Problem 1
Consider triangle
Rotate triangle
around the midpoint of side . Right click on the point and select Rename to label the new vertex . Rotate triangle
around the midpoint of side . Right click on the point and select Rename to label the new vertex . Look at angles
, , and . Without measuring, write what you think is the sum of the measures of these angles. Explain or show your reasoning. Is the measure of angle
equal to the measure of any angle in triangle ? If so, which one? If not, how do you know? Is the measure of angle
equal to the measure of any angle in triangle ? If so, which one? If not, how do you know? What is the sum of the measures of angles
, , and ?
Print Version
Here is triangle
Rotate triangle
around the midpoint of side . Label the new vertex . Rotate triangle
around the midpoint of side . Label the new vertex . Look at angles
, , and . Without measuring, write what you think is the sum of the measures of these angles. Explain or show your reasoning. Is the measure of angle
equal to the measure of any angle in triangle ? If so, which one? If not, how do you know? Is the measure of angle
equal to the measure of any angle in triangle ? If so, which one? If not, how do you know? What is the sum of the measures of angles
, , and ?
Activity 2: Every Triangle in the World
Problem 1
Here is
What is
? Explain how you know. Use your answer to explain why
. Explain why your argument will work for any triangle: that is, explain why the sum of the angle measures in any triangle is
.
Are you ready for more?
Problem 1
Using a ruler, create a few quadrilaterals. Use a protractor to measure the four angles inside the quadrilateral. What is the sum of these four angle measures?
Come up with an explanation for why anything you notice must be true (hint: draw one diagonal in each quadrilateral).
Activity 3: Four Triangles Revisited
Problem 1
This diagram shows a square
Given that angle
Lesson Summary
Using parallel lines and rotations, we can understand why the angles in a triangle always add to
A 180 degree rotation of triangle