theorem Th86: :: SIN_COS9:86

for x being Real
for f being PartFunc of REAL,REAL st f is_differentiable_in x & f . x > - 1 & f . x < 1 holds
( arccot * f is_differentiable_in x & diff ((arccot * f),x) = - ((diff (f,x)) / (1 + ((f . x) ^2))) )