How many ways can four points be arranged in a plane so that the six distances between pairs of points take on only two different values?
a) 4 of the 5 points which form a pentagon.
b) An equilateral triangle ABC + a point D outside the triangle which
is on the bisector of angle A, at a distance such that AD = AB. D
can be on the near or far side of the triangle, yielding two different
shapes
And three more
