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.
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There are 5 lines with slope 0 , 5 lines of the same positive slope, and 5 lines of the same negative slope. Any triangle must include exactly one line of each slope. Now out of the 2 5 intersections between the lines with positive and negative slopes, 6 intersect without a third common intersection with a horizontal line and 1 9 do. So if we choose one of the 6 pairs of lines without a third common intersection for a triangle, then we have a choice of 5 horizontal lines for a third side otherwise there are only 4 horizontal lines to choose from. Thus the answer is 6 ( 5 ) + 1 9 ( 4 ) = 1 0 6 .