Question

m=f/f+u formula derivation
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Answer 1

1/v + 1/u = 1/f

1/v = 1/f - 1/u

1/v = u - f / uf

v = uf / u - f --------(1)

m = - v / u -------(2)

substitute (1) in (2)

m = - ( uf / u-f ) / u

m = - uf / (u-f)*u

m = -f / (u-f)

m = - f / - ( -u + f )

m = f / f - u