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I made one small triangle
$3$ matches |
$9$ matches |
I added another row and counted the number of small triangles and counted the matches.
Find a good way to record your results. See if you can predict the numbers for rows of triangles you have not drawn.
When you have done all you can with triangles, see if you get the same sort of results with squares. Then think of other shapes which might make number patterns as they grow.