Drawing a Circumcircle.

How to…

Step 1

• Starting with a triangle with corners A, B and C, construct the perpendicular bisector of AB.

• To do this, set your compasses to slightly more than half the length of AB, put the point of the compasses at A, draw two arcs either side of AB, move the pint of the compasses to B draw two more arcs cutting the first two, then join the crossing points.

Step 2

• Construct the perpendicular bisector of the edge AC. (Use the same method as before, but be careful not to muddle up your arcs from step 1 and your new arcs!)

Step 3

• Construct the perpendicular bisector of the edge BC.

• You need to concentrate to make sure you don’t mix up the arcs.

• If you have been accurate your three perpendicular bisectors should all meet at the same point.

Step 4

• To draw the circumcircle, put the point of the compasses where the three perpendicular bisectors cross.

• Open the compasses so the pencil point is at point A.

• Draw the circle. If you have been accurate up to now, the circle should pass through each of the points of the triangle.

This tests…

• How well you have understood how to construct a perpendicular bisector of a line.

• How accurate you are when drawing.

• How well you can concentrate on, and follow a set of instructions.