A, B, C, D*** Circle, draw this circle.

Take the midpoint m of the arc CD,

Angle DBM= Angle CBM= 1/2 Angle DBC=90- Angle ABD.

= & gt

Angle ABM = 90

= & gt

AM is the diameter of a circle.

Lian DM, CM

Angle ADM= angle ACM = 90

DM=CM

AM=AM

ADM equals ACM

Angle AC=AD