BD is a diameter of circle O. If m∠ACB = 30°, what is m∠DBA?

it is 60

m∠DBA = 60º

Step-by-step explanation:

Angle subtended by the diameter is 90º:

⇒ m∠DAB = 90º

Angles in a circle formed by the same arc are equal:

⇒ ∠ ADB = ∠ACB

⇒ m∠ ADB = 30º

Sum of angles in a triangle is 180º:

⇒ m∠DBA + m∠DAB + m∠BDA = 180º

⇒ m∠DBA = 180° - 90° - 30°

⇒ m∠DBA = 60°