theorem :: DIFF_4:20

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