theorem :: DIFF_4:19

for h, x being Real st x + h in (dom cosec) /\ (dom sec) & x in (dom cosec) /\ (dom sec) holds
(fD ((cosec (#) sec),h)) . x = - (4 * (((cos ((2 * x) + h)) * (sin h)) / ((sin (2 * (x + h))) * (sin (2 * x)))))