1V 1F - 1U. So A is to B as f is to di minus the focal length. D1p Parallax is a method of using two points of observation to measure the distance to an object by observing how it appears to move against a background.
The parallax formula states that the distance to a star is equal to 1 divided by the parallax angle p where p is measured in arc-seconds and d is parsecs.
So this is di minus the focal length. To find the distance to the Cepheid. The lens equation is 1f 1do 1di where f the focal length of the lens. Calculation of Focal length Image distance and Object distance are made easier using this mirror equation calculator.