∠B in ΔABC and ∠B in ΔDBC is 90°. BC is a common side in both triangles which mean that both triangles have one side of the same length. Side AC in ΔABC is the same length as side DC in ΔDBC. Therefore ∠C in both ΔABC and ΔDBC are the same size. Therefore ΔDBC is a mirror image of ΔABC, which is a form of congruent triangles.
