How Many Triangles?

How many triangles are in the figure below?

Note a triangle must have sides made up of the lines shown in the figure, that is the image border does not contribute to making triangles.


The answer is 106.

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1 solution

Ron Nissim
Jun 3, 2018

There are 5 5 lines with slope 0 0 , 5 5 lines of the same positive slope, and 5 5 lines of the same negative slope. Any triangle must include exactly one line of each slope. Now out of the 25 25 intersections between the lines with positive and negative slopes, 6 6 intersect without a third common intersection with a horizontal line and 19 19 do. So if we choose one of the 6 6 pairs of lines without a third common intersection for a triangle, then we have a choice of 5 5 horizontal lines for a third side otherwise there are only 4 4 horizontal lines to choose from. Thus the answer is 6 ( 5 ) + 19 ( 4 ) = 106 6(5)+19(4)=\boxed{106} .

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