Let O be a point outside a given circle. Two lines OAB,OCD through O meet the circle at A,B,C,D, where A,C are the midpoints of OB,OD, respectively. Additionally, the acute angle θ between the lines is equal to the acute angle at which each line cuts the circle. Find cosθ and show that the tangents at A,D to the circle meet on the line BC. trigonometrygeometry unsolvedgeometry