A typical red blood cell subtends an angle of only 1.90E-5 rad when viewed at a person's near-point distance of 25.0 cm. Suppose a red blood cell is examined with a compound microscope in which the objective and eyepiece are separated by a distance of 12.5 cm. Given that the focal length of the eyepiece is 2.56 cm, and the focal length of the objective is 0.500 cm, calculate the magnitude of the angle subtended by the red blood cell when viewed through this microscope. 1/f=1/di+1/do i dont really know how to start this problem. help would be appreciated.