6. Circle Tool and Compass

Two most common tools for drawing circles are Circle tool and Compass. This lesson explains the difference.

6.1. Circle Tool and Compass

If you want to draw a circle with a given center and going through another given point, use the Circle Tool . It requires you to point at two points, one of which is the center and another is the point on the circle. The order is important - see hints in the status line to find out which one goes first. (The light red line outlines the final objects that you are supposed to draw.) Text continues below.

If you want to draw a circle with known center and radius, use Compass (it is called Circle from 3 points in these java applets). You use this tool as you would use a traditional compass: first you need to measure the radius with your compass and then you move it over (with the fixed span) to the center of the circle.

To draw a circle the center of which is X and radius KL, select Compass . Now you have to point to 3 points - two determine the radius and one the center. See the hints in the status line for more information. If you constructed both circles correctly,  "Well Done" message window will show.

Using compass in Cabri is a little bit faster - it suffices to point to the line segment as a whole; you do not have to point to each endpoint of the line segment.

6.2 Six circles (practice)

Construct the following 6 circles:

1. c1: center is A, goes through B

2. c2: center is B, goes through C

3. c3: center is C, radius is KM

4. k1: center is X, radius is LM

5. k1: center is X, radius is KM

6. k1: center is X, radius is KL

If your construction went well and you got the "Well Done" message, try to grab and move points K, L, M. Observe what happens. Explain why some (and only some) circle change.