Fermat’s principle to derive lens formula

1. For the past 1 hour, I’m trying to derive lens maker’s equation using fermat’s principle, which of course is our homework. Any help would be great regarding the topic.

2. According to Fermat’s Principle, optical path length OPL = PA + AQ, here PA and AQ are two rays. Now using this I have to derive Lens makers Formula.

3. Now the diagrams and eqn’s i don’t know how to put in here. Upto the point where I found PA + AQ, I have completed.
PA = (x^2 + h^2)^1/2 = x + 1/2 X h^2/x
AQ = (y^2+ h^2)^1/2 = y + 1/2 X h^2/y
Now, PA + AQ = x + y + h^2/2 X (1/x + 1/y)
Now kindly help me if you happen to know the next steps.


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