2 Triangles in 1

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SWBAT prove that a line parallel to one side of a triangle divides the other two proportionally while using AA criterion.

Big Idea

Students will complete a discovery activity to uncover key properties of similar triangles and apply knowledge of AA criterion.

Do Now

10 minutes

In this Do Now, students will have an opportunity to review solving proportions.  In this lesson, students will be required to set-up and solve proportions to find the lengths of sides (and parts of sides) of triangles. 

You can ask students to come to board and share their work (MP 3) on this Do Now as it is a pre-requisite skill. 


Avatar Video and Exploration

30 minutes

I would suggest that the last 4 minutes of the video are most useful in starting a conversation with students about similarity.  Most of my students had seen Avatar so the introductory material may be unnecessary if you are limited on time. 

Activity/Homework and Exit Ticket

15 minutes

In the Activity-Homework for this lesson, students will work on using concepts about similar triangles and proportionality to complete practice questions.  The questions for #6-9 are great problems to review with students since #6-8 review using the concepts of proportionality to find length measures in triangles.  Question 9 requires students to look for regularity in repeated reasoning.  Students must remember that when two consecutive interior angles are supplementary, it means that lines are parallel (MP 8).


The Exit Ticket can used use as a formative assessment to ensure that students understand the key concepts in today's lesson.  This is particularly important since the lesson after this, Similiarity Applied, will ask students to persevere through more challenging questions.