There is something wrong with the topic, which should be that CD is the height of AB on the side of △ABC, satisfying: CD 2 = AD× BD, and verifying ∠ ACB = 90 is the eighth question of eighth grade mathematics in China Normal University Edition, P 12 1.

CD=CA+AD=BD+CB

De: CA=CD-AD，CB=CD-BD。

So: ca× CB = (CD-AD )× (CD-BD) = Cd2-AD× CD-BD× CD+AD× BD.

CD is the height of AB, so ad×CD = BD×CD = 0；; CD^2=AD×BD

So CA×CB=0, so CA and CB are perpendicular to each other, that is, ∠ ACB = 90.