I'm afraid that's not coreect.

Thanks CPhill i understand now!

Thanks, but that answer is incorrect. I haven't learned tan and arctan yet, do you have another solution? Thanks again.

Since this is an isoceles trapezoid, it is impossible for OA=OB=OC=OD=r. However, OA = OB, OC = OD is true. Also line EO is the radius.