just a quick addition to the great reply above, Check the picture below.
notice on the picture atop, if we lay the 14 side on top of the 20 side, the slack is just 6, however we get a flat-line not a triangle, so the red side must be greater than 6 for that.
on the picture below, if we had the two small sides to open to a flat line, they add up to 17, no way to fit a side of 20 there, so the red side must be less than 17.
