I already derived it
Once again check
Consider a convex lens (or concave lens) of absolute refractive index m2 to be placed in a rarer medium of absolute refractive index m1.
Considering the refraction of a point object on the surface XP1Y, the image is formed at I1 who is at a distance of V1.
CI1= P1I1 = V1 (as the lens is thin)
CC1 = P1C1 = R1
CO = P1O = u
It follows from the refraction due to convex spherical surface XP1Y
The refracted ray from A suffers a second refraction on the surface XP2Y and emerges along BI. Therefore I is the final real image of O.
Here the object distance is
(Note P1P2 is very small)
(Final image distance)
Let R2 be radius of curvature of second surface of the lens.
\ It follows from refraction due to concave spherical surface from denser to rarer medium that
Adding (1) & (2)
hope it helps