Bonjour ;
[tex] f(x) = (x + \dfrac{1}{4} ) e^{-4x} + b \ donc \ \ : \ f'(x) = (x + \dfrac{1}{4} )'e^{-4x} + (x + \dfrac{1}{4} )(e^{-4x} )' \\\\\\ = e^{-4x} + (x + \dfrac{1}{4} )\times (-4x)'e^{-4x} = e^{-4x} -4 (x + \dfrac{1}{4} )e^{-4x} \\\\\\ = (1 - 4x - 1)e^{-4x} = - 4xe^{-4x} . [/tex]