A convex lens of focal length f is placed somewhere in between an object and a screen. The distance between the object and the screen is x. If the numerical value of the magnification product by the lens is m, What is the focal length of the lens ?
x = u + v $ m = { f \over ( f + u ) } = { (f - v ) \over f } $ $ u = { -(m+1) \over m } f $ $ v = (m+1) f $ solved the equation