Question

I need help with this problem it

Answer 1

`(1)/(u) + (1)/(v) = (1)/(f) \\ \\ (1)/(u) = (1)/(f) - (1)/(v) \\ \\ (1)/(u) = (1)/(f) ( (v)/(v) )- (1)/(v) ( (f)/(f) ) \\ \\ (1)/(u) = (v)/(vf) - (f)/(vf) \\ \\ (1)/(u) = (v - f)/(vf) \\ \\ u = (vf)/(v-f)`