In the given figure AB is the diameter of the circle
Angle CAD = 30 degree and Angle CBA = 70 degree
Angle ACD = ?
Drawing lines DB and AC in the figure
now Angle CAD = Angle CBD = 30 degree ( Since a chord of a circle subtends equal angles on all its circumference)
⇒ Angle DBA = 40 degree
now Angle DBA = Angle ACD = 40 degree ( Since a chord of a circle subtends equal angles on all its circumference)