Answer:
f = uv/ (v+u)
Explanation:
1/f= 1/u+1/v
Multiply each side by fuv to clear the fractions
fuv( 1/f)= fuv(1/u+1/v)
uv = fv+ uf
Factor out the f on the right hand side
uv = f(v+u)
Divide each side by (v+u)
uv/ (v+u) = f(v+u)/ (v+u)
uv/ (v+u) = f