In this diagram of circle C, m∠BCD is 18° and the m is 33°. Complete this explanation about how to find the measure of arc ABD. If m = 33°, then m∠ECF = _° because the measure of a _ angle is always equivalent to the arc it intercepts. If m∠ECF = _°, then m∠ACB = _° because they are vertical angles. If m∠ACB = _° and m∠BCD = _°, then m = 33° and m = _°. By the Arc Addition Postulate, m= m + m, so m= _°.