Wednesday, January 27, 2016

Building with Desmos: How I Spent My Third Consecutive Snow Day

We just recorded the biggest snowfall in the Lehigh Valley ever (over 30 inches) and despite clear roads, we are still having snow days! There are narrow roads in city portions of our area and there is too much snow for busses to make the turn onto some of those roads.  This excessive (and unplanned) time off from work gave me time to think and to build something really cool in Desmos Activity Builder.  If you haven't tried Desmos Activity Builder out yet and you are a math teacher, you are missing out.  This is my 4th lesson built in Desmos Activity Builder.

The lesson focused on tangents to circles.  In the past we would look at two big ideas, as illustrated below.

Big Idea #1: Tangents are perpendicular to radii at the point of tangency.

Big Idea #2: Tangent segments to a circle from a common point are congruent.

Usually we would discover these ideas using Geogebra or hands-on activities.  However, I don't want my students to forget concepts from Algebra, such as equations of lines and I realized that with Desmos we could look at these concepts through the lens of coordinate geometry.  Plus, I had time and welcomed the challenge of creating the activity in desmos.  And it was quite challenging.  (If you are interested in the details, let me know by posting a comment and I can send you the file with the hidden folders.)

What does the activity do and what does it look like?  I could tell you, but that would ruin the fun of trying it our for yourself. is the link: Tangents to Circles.

My students will be doing this activity in class on Monday (February 1st) and I will post back to this blog with a few comments of how things went.  Be sure to return next week.


  1. I love this! Not doing circles for quite a while yet, but I'm definitely going to borrow!!

  2. Thanks! The activity went well and I made some modifications to some of the sides thanks to input from my department and fellow AP Stat teacher, Bob Lochel! The kids get so much more out of a concept and remember it better when they have time to play around with it.