Khan Academy
Khan Academy: Geometry: More on Why Ssa Is Not a Postulate
Explains why SSA is not a valid postulate for proving triangle congruence. Practice problems available. This video [7:24] also appears in the Geometry: Triangles strand.
Khan Academy
Khan Academy: Geometry: Two Column Proof Showing Segments Are Perpendicular
In this video tutorial [11:06], the instructor shows how to construct a two-column proof using triangle congruence postulates to prove that two line segments are perpendicular.