Perp Lines are Perfect

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SWBAT find the perpendicular bisector of a line segment and construct perpendicular bisectors using a compass and straight-edge.

Big Idea

Algebra and geometry intersect, please pardon the pun, in this lesson on perpendicular bisectors.

Do Now

10 minutes

Students will complete two warm-up questions that require them to use pre-requisite skills of writing equations of perpendicular lines and also finding the midpoint of a line segment.

Constructions with Perpendicular Lines

25 minutes

The student notes provide scaffolded notes and examples for students to complete two constructions of perpendicular lines through a point on a line and a point not on the line.  These videos explaining construction from a point on a line and not on a line are a great resource for in-class instruction and also for absent students (MP 5). 

CCSS for Geometry requires that students prove how and why constructions work.  The construction in this lesson requires students to have knowledge of congruent triangles (the next unit) so teachers should make students that we'll be proving why these constructions work in future lessons (MP 7). 


10 minutes

Student can work on practice question in pairs or small groups.  To review, you can ask students to explain how the 3 constructions learned in class today are similar and different.  One suggestion is that you may want to ask students to come re-state the steps to these constructions in their own words. 

Exit Ticket

10 minutes

Students will find the perpendicular bisector of a line segment given 2 endpoints.  This Exit Ticket can easily be collected by asking students to write their work on a separate piece of paper or a post-it note.