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## Main Question or Discussion Point

I was practicing hw and I came across these two derived equations from 1/f=1/d0+1/di

1st one 1/di=1/d0-1/f

2nd one di=d0(f)/d0-f

How do distinguish which one to use? and how did they get that derivation for the 2nd equation?

1st one 1/di=1/d0-1/f

2nd one di=d0(f)/d0-f

How do distinguish which one to use? and how did they get that derivation for the 2nd equation?