The vertex where the two triangles touch forms a pair of vertical angles, so that the angles on opposites sides of the vertex are congruent. You're shown that two legs of both triangles are congruent. All this tells you that the two triangles are isosceles, and furthermore that they are similar because of the congruence of their "base" angles.
This means




I think it is, I have no clue.
Here's some examples though, 5 example of not well defined sets: - Number of fire damages. - Number of reported crime scenes. ... - Numbers of illegal settlers.
Red, blue, yellow, green, purple is well-defined since it is clear what is in the set.