Question

1/f=1/u+1/v. find u in terms of f and v

Answer 1

1/u=1/f - 1/v
1/u= v-f/vf (taking lcm)
u = vf/v-f

Answer 2

1/f=1/u+1/v
1/u=1/f-1/v
1/u=v-f/fv
inverse on both sides
u=fv/v-f
hope it helps you