Effective focal length of two plano convex

[delgado]

Homework Statement

Hello
Two plano convex with f1=f2=30mm and R1=15.45mm and R2=-R1. The two plano convex have the same thickness too (3.16 mm) and their curves colide in the propagation axis.
I believe you understand what i mean
What is the focal length of the two plano convex system?

Thanks in advance

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

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