standard_description: Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.
problem_activity: Activity
The purpose of this activity is for students to match relationships between angles in a figure with equations that can represent those relationships. This prepares students for writing and solving equations that represent relationships between angles in the next lesson. As students explain their reasoning, monitor for students who use the appropriate vocabulary and language (i.e. vertical angles have equal measures, supplementary angles sum to 180 degrees, etc.).
Launch
Keep students in same groups. Give students 2–3 minutes of quiet work time followed by partner and whole-class discussions.
Student Facing
Match each figure to an equation that represents what is seen in the figure. For each match, explain how you know they are a match.
[IMG]
\(g+h=180\)
\(g=h\)
\(2h+g=90\)
\(g+h+48=180\)
\(g+h+35=180\)
Student Response
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Student Facing
Are you ready for more?
What is the angle between the hour and minute hands of a clock at 3:00?
You might think that the angle between the hour and minute hands at 2:20 is 60 degrees, but it is not! The hour hand has moved beyond the 2. Calculate the angle between the clock hands at 2:20.
Find a time where the hour and minute hand are 40 degrees apart. (Assume that the time has a whole number of minutes.) Is there just one answer?
Student Response
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Activity Synthesis
The goal of this discussion is for students to articulate the angle relationships they noticed in each figure and equation. Display the figures for all to see. Select previously identified students to share their explanations for each figure. If not mentioned in students’ explanations be sure that students see the vertical, supplementary, straight, and right angle relationships in the figures.


aligns: no