Hart's A-frame
■ Generalized A-frame (= variation)
if AD = c, DC = d,
same matter can be realized, c ≠ a, d ≠ b.
ab = cd condition is OK.
--- why? ----- Do consider by yourself. (Denken Sie nach.)
cf. Generalized Hart's A-frame --- my sample
a + b ≠ c + d, so, it's not an isosceles triangle.
trace of node C is ⊥ AB. --- true.