give the derivation of lens maker formula
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https://www.meritnation.com/ask-answer/question/derive-lens-maker-formula-by-concave-lens/ray-optics-and-optical-instruments/7490225
https://www.meritnation.com/ask-answer/question/derive-lens-maker-formula-by-concave-lens/ray-optics-and-optical-instruments/7490225