Nine-pin Triangles
Seven possible in total. If number pegs as 1-9, then can describe
them:
Four with "base" of 1 i.e. joining adjacent dots: 1, 2, 3; 1, 2, 4;
1, 2, 5; 1, 2, 6
Two more different ones with "base" of 2: 1, 3, 5; 1, 3, 1, 3,
6
One more different with "base" of 3: 1, 4, 7
This problem offers an opportunity for pupils to work in a
systematic way, using their knowledge of the properties of
triangles. A useful discussion about which triangles are the same
and which are different could be encouraged. If working on paper
rather than using the interactivity, pupils may find it helpful to
print this sheet off.