1. The problem statement, all variables and given/known data
erm the general idea is right but i guess your derivatives are wrong :)
you should get
ΔN = sqrt [ ( (-a/y)*exp(-x/y)*Δx )² + ( (ax/y²)*exp(-x/y)*Δy )² ]
y and x interchanged in first term :)
i hope that solves your problem otherwise i don´t know what´s wrong there the gauß formula is ok as far as i see :)
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