Day 43: Constructing Parallel Lines

Today in Geometry I asked students to draw a line then create another parallel to it.

There were a lot of crazy constructions.

Some students drew a line then used the opposite side of the straight edge to construct a parallel line. Others copied angles or used an insane amount of circles. Here’s one of those methods:

*Students were using a straight edge and compass

Start with a line segment


Construct a circle whose radius spans more than half the segment.


Copy that same circle, this time centered and the other endpoint. (they just made a perpendicular bisector…)


Copy the same circle again, but make it just touch point B and do the same for the A side also.

Day44.3Connect the first intersection of the circles to the third.


There are a few points that I am uneasy with here but this particular student created a parallel line; doing exactly what they were asked to do. They also created an isosceles trapezoid, which is pretty awesome.

Here is the big question… if this method were used on a test would you mark it correct?


2 thoughts on “Day 43: Constructing Parallel Lines

  1. Yes, because it is correct. I found students doing this as well. The challenge comes when you ask them to construct a parallel through a defined point. That tends to drive them toward some of the more traditional approaches, and it is often the first time they really get that two perpendiculars can make a parallel:)


    1. I held back from commenting on their different approaches. We are construction a parallel through a point today, it will be interesting to see how students modify their constructions! Thanks for stopping by!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s