Task id16334

The segment AB is located along a straight line passing through the focus of the collecting lens at an angle a = 60° to its main optical axis. The distances from points A and B to the focus F are equal, respectively: a = 5 cm, b = 10 cm. What is the focal length of the lens F, if it is known that the length of the segment AB is equal to the length of its image (Fig.a).