MethodDraw a pentagon, a hexagon, a heptagon and an octagon on chart paper, one per group. They do not have to be neat or regular. Pick one corner of each and rule lines to every corner it is not already joined to. Count the triangles and fill in the table. Do not use the formula yet.
Then find the pattern yourselvesPut all four groups results on the board together. Look down the sides column and the triangles column. Somebody will see that the triangles are always two fewer. Only once the class has said it out loud should the formula be written down.
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