In the circle below, AB || CD, AD is a diameter of the circle, and AD = 36. What is the number of inches in the length of arc AB? Express your answer in terms of pi.
Let the center = O
Then OA = OB so in triangle OAB angles BAO and ABO are equal = 60°
So angle OAB also = 60°
And arc AB = 60° = 1/6 of the circle's circumference
The radius of the circle = 18
So arc AB = (1/6)* 2 * pi ( 18) = 6 pi