Properties of Parallelograms and Special Parallelograms

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Students will be able to apply angle relationships and properties of isosceles triangles and trapezoids in a proof.

Big Idea

In the Logical Road Map, students will make sense of multiple pathways to writing a proof.

Warm-Up: Logical Road Map

25 minutes

I like using the Logical Road Map for today’s lesson because we will spend time and detail on justification and proof writing.  There are several ways to write the proof for the logical road map—many students will use corresponding and alternate interior angles to write the proof.  Since this unit is on discovering and proving polygon properties, I make sure to look out for students who use properties of parallelograms and isosceles trapezoids to write their proofs, having them display their work under the document camera for the class to see. Viewing and discussing alternative proofs enables students to make connections among concepts (MP3).   

Homework Review: Focus on Proof

25 minutes

It is important for me to give adequate time to review last night's homework assignment:

Prove that if a trapezoid has congruent base angles, it must be isosceles. 

These are often the most difficult proofs for my students. Students start with seemingly nothing (no diagram, for example), but they are required to prove a rather important idea. 

I like to have at least two student volunteers present their proofs (or ideas for how to write the proof) to the whole class.  Ultimately, someone will see that one of the ways to write the proof is directly related to the Logical Road Map Warm-Up.  Ideally, others will see that adding auxiliary lines (two perpendiculars to the bases) can allow them to use properties of rectangles to write the proof, or that extending the sides of the trapezoids that are not the bases can allow them to use properties of isosceles triangles to write the proof.  

The Common Core calls students to construct viable arguments and critique the reasoning of others, which is why it is important to allow students to discuss multiple ways to write the proof (MP3) and to give feedback to improve the quality of these proofs, which requires them to attend to precision (MP6).

Notes: Parallelograms and Special Parallelograms

20 minutes

After beginning the class with a focus on proofs, we take brief reminder notes on the properties of parallelograms and special parallelograms. My students often need a quick review of these properties. Today, they will apply the properties to solve leveled problems in a Group Challenge during tomorrow's lesson

Next, I introduce the Parallelogram Properties Umbrella to students.  Since classification and differentiation often pose challenges to students, this is a good time to give students an umbrella graphic organizer to help them organize their thinking hierarchically. The use of a graphic organizer better enables my students to see the relationships that exist within the parallelogram family.  Encouraging students to see rhombuses as equilateral parallelograms, rectangles as equiangular parallelograms, and squares as both equilateral and equiangular parallelograms, is an important learning objective at this stage of my course.  

Practice: Determining Parallelograms

20 minutes

I like to close this lesson with the Determining Parallelograms worksheet. The task is tricky. It requires students to apply all properties of special parallelograms and to think about proofs before actually writing them. I find that this worksheet often helps my students to self-assess their understanding of parallelogram and special parallelogram properties.  

By the time that they complete this worksheet, my students prove several theorems about parallelograms. One of the problems gives them the opportunity to prove that if a quadrilateral has one pair of congruent, parallel sides, it must be a parallelogram.