theorem :: FDIFF_3:10

for f1, f2 being PartFunc of REAL,REAL
for x0 being Real st f1 is_left_differentiable_in x0 & f2 is_left_differentiable_in x0 holds
( f1 + f2 is_left_differentiable_in x0 & Ldiff ((f1 + f2),x0) = (Ldiff (f1,x0)) + (Ldiff (f2,x0)) )