How many triangles are there in the figure below?
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Nice problem. :) There are 56 triangles total.
Ashish posted a clear breakdown of where they are in 4 categories. However, here is an exhaustive proof that no additional triangles exist. Ashish, your method of counting is certainly faster. However, the technique I used is cool in that it will work for any triangle counting problem.
Step 1 : Identify each vertex and, for convenience, group them into equivalent classes by symmetry.
Step 2 : For each vertex, consider every possible pair of lines extending out from it. Again, group for symmetry.
Step 3 : For each pair of lines extending from a vertex, a triangle exists if and only if a point on one line is connected by a line in the diagram, directly to a point on the other line. Counting these identifies every triangle that the selected vertex is part of.
Step 4 : Add up all of the triangles counted by this method and, finally, divide this sum by three because each has been triple counted (once for each vertex.)