pls derive the formula m = f / (u+f)

1/v - 1/u = 1/f

so 1/v = 1/f + 1/u

1/v = (u+f)/uf

v = uf/(u+f) eq no.1

magnification for lens = m = v/u eq no.2

substituting 1st equation in 2nd we get, m = uf/(u+f) x 1/u

when u cancel u from numerator and denominator, u get, m = f/(u+f)

hence proved..

Hope u got it.. :D