theorem :: DIFF_3:61

for x0, x1 being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = 1 / (sin x) ) & sin x0 <> 0 & sin x1 <> 0 holds
[!f,x0,x1!] = - (((2 * ((sin x1) - (sin x0))) / ((cos (x0 + x1)) - (cos (x0 - x1)))) / (x0 - x1))