theorem Th14: :: NDIFF_4:14

for n being non zero Element of NAT
for Z being open Subset of REAL
for f being PartFunc of REAL,(REAL n) st Z c= dom f & f is_differentiable_on Z holds
( - f is_differentiable_on Z & ( for x being Real st x in Z holds
((- f) `| Z) . x = - (diff (f,x)) ) )