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'Triangle Pin-down' printed from http://nrich.maths.org/
Have you looked at the angles in the triangles drawn? It might help
to draw in some radii and work out angles from the centre.
How will you record the triangles you can make? You might like to
print off these sheets if you're not using the interactivity:
Sheet of three-peg
Sheet of four-peg
Sheet of five-peg
Sheet of six-peg
Sheet of seven-peg
Sheet of eight-peg
Sheet of nine-peg