I have been reading in multiple places (e.g. [1], section 4) that the VC-dimension of the class of triangles (in 2D space) is 7. The issue is that, for the case when 4 points lying on a straight line with alternating signs (like - + - +), I don't see why a triangle could separate the points. Where am I making mistake?
[1] http://www.igi.tugraz.at/lehre/CI/SS08/tutorials/VC_examples/VC_examples.pdf