Again, those triangles would be right angled.OR
AC divides BD into 2 equal parts
Those angles being equal can have the angles ADC and ABC bothOR
The angels ACB and ACD are equal ?
in which case the quadrilateral would not be cyclic,
since opposite angles would not necessarily sum to
Angles ADC and ABC either have to be right angled, or summing to
Having them sum to is required.