Question

How do you solve for di in `L/f= L/(di)+L/(do)`?

Answer 1

You do simple algebra.

Explanation:

We want di so we isolate the part containing di
(L/f)-(L-do)=(L/di)
Then we take the opposite of every fraction so we get di in the numerator's position
1 / (L/f)-(L/do) =di/L
and we multiply by L
L/ (L/f)-(L/do)=di.