1÷u+1÷v=1÷f. Rearrange this to make u the subject.?

Multiply by uvf to begin.

1÷u + 1÷v = 1÷f
vf + uf = uv
uv - uf = vf
u = vf / (v - f)

1÷u+1÷v=1÷f
1/u + 1/v = 1/f
1/u = 1/f - 1/v
1/u = (v - f)/(fv)
u/1 = fv/(v - f)
u = fv/(v - f), where u, v and f ≠ 0, v ≠ f

1/u + 1/v = 1/f

f/u + f/v = 1

fv + fu = uv

uv - fu = fv

u(v - f) = fv

u = fv/(v - f)

1/u + 1/v = 1/f
1/u = 1/f - 1/v
1/u = (v-f) / (fv)
u = fv / (v-f)

1÷u+1÷v = 1÷f
(1/u) + (1/v) = 1/f
1 + u/v = u/f
(u/f) - (u/v) = 1
u(1/f - 1/v) = 1
u(v-f)/(fv) = 1
u = fv/(v-f)