for x0, x1 being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = 1 / ((cos x) ^2) ) & x0 <> x1 & cos x0 <> 0 & cos x1 <> 0 holds
[!f,x0,x1!] = ((((((- 16) * (sin ((x1 + x0) / 2))) * (sin ((x1 - x0) / 2))) * (cos ((x1 + x0) / 2))) * (cos ((x1 - x0) / 2))) / (((cos (x0 + x1)) + (cos (x0 - x1))) ^2)) / (x0 - x1)