theorem Th31: :: DIFF_3:31

for h, x being Real
for f1, f2 being Function of REAL,REAL holds ((bdif ((f1 (#) f2),h)) . 1) . x = ((f1 . x) * (((bdif (f2,h)) . 1) . x)) + ((f2 . (x - h)) * (((bdif (f1,h)) . 1) . x))