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Triangles in Circles

Age 11 to 14
Challenge Level

Triangles in Circles printable sheet
9 dot circles printable sheet

In the GeoGebra interactivity below there is a circle with 9 equally spaced points on the edge, and one in the centre.

Draw as many different triangles as you can, by joining the centre dot and any two of the dots on the edge.

Can you work out the angles in your triangles?

You should have found four different triangles with angles of:
40, 70, 70
80, 50, 50
120, 30, 30,
160, 10, 10

Here is a triangle formed by joining three dots on the edge of the nine-point circle:


Can you work out the angles of this triangle?
Click to reveal a diagram that might help you work out the angles.

Create some more triangles by joining three dots on the edge of the nine point circle.
How many different triangles can you make?
Can you work out the angles each time?

When the centre dot isn't inside your triangle, you might find it a little trickier to work out the angles. Click below for a diagram that might help:


Many thanks to Geoff Faux who introduced us to the merits of the 9 pin circular geo-board.