CPSTP = Corresponding parts of similar triangles are proportional.

        ~  = Similar

Right Triangles ABC ~ ADC ~ DBC                   <ABC = <DBC,<CAB = <BCD

Triangles with = angles are similar.

AC/AD  = AB/AC, AC^2  =  ABAD                  CPSTP      

CB/DB  =  AB/CB, CB^2  =  ABDB                   CPSTP

Adding  these we get  AC^2 + CB^2  =  AB(AD+DB) = AB^2