Andy C. answered • 06/13/18

Math/Physics Tutor

angles CDB and CBD are congruent because of the isoceles triangle BCD

Likewise, angles ABC and BCA are congruent because of isoceles triangle ABC

So BCD and BAC are congruent by 180 degree property of triangles

Since the corresponding angles are congruent, the triangles are similar

SPECIFICALLY, triangles BCD and CAB are similar

The sides are in proportion

DB / CB = CB / AB

Cross multiplication completes the proof