Signal to noise ratio and uncertainty in magnitude

[CGM]

1. The problem statement, all variables and given/known dat
Show that if you have a signal f and noise σ_f for a stellar flux measurement, then the uncertainty on the observed magnitude is given by
σ_m = 1.0875 x σ_f / f

Homework Equations

(you should use the fact that when ε<<1 then log(1±ε) ≈ ε/ln10 ).

The Attempt at a Solution

 

[RUber]

You should show some attempt at the problem in this forum.
Start with: let signal + noise be $f + \sigma_f = f(1 + \frac{\sigma_f}{f} )$
Do you know how to calculate the uncertainty?

 

[CGM]

You should show some attempt at the problem in this forum.
Start with: let signal + noise be $f + \sigma_f = f(1 + \frac{\sigma_f}{f} )$
Do you know how to calculate the uncertainty?

Uncertainty in (1 + σ_f/f) = √((δσ_f/σ_f)² + (δf/f)²)
?
or uncertainty in f + σ_f =√(δσ_f)² + (δf)²