Similar triangles are a special case of similar polygons,
so one should prove triangles similar in exactly the same way as for polygons
in general.
However, the interesting thing is that in the special case of triangles
(and this applies only to triangles), it is not necessary to
prove both conditions, because it can be proved that the one
implies the other.
Two triangles are
similar iff:
1. all the corresponding
angles are equal, or
2. all the corresponding
sides are proportional.
