let Z be open Subset of REAL; :: thesis: ( Z c= dom (cot (#) sec) implies ( cot (#) sec is_differentiable_on Z & ( for x being Real st x in Z holds
((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) ) ) )
assume A1: Z c= dom (cot (#) sec) ; :: thesis: ( cot (#) sec is_differentiable_on Z & ( for x being Real st x in Z holds
((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) ) )
then A2: Z c= (dom cot) /\ (dom sec) by VALUED_1:def 4;
then A3: Z c= dom cot by XBOOLE_1:18;
for x being Real st x in Z holds
cot is_differentiable_in x

proof

let x be Real; :: thesis: ( x in Z implies cot is_differentiable_in x )
assume x in Z ; :: thesis: cot is_differentiable_in x
then sin . x <> 0 by A3, FDIFF_8:2;
hence cot is_differentiable_in x by FDIFF_7:47; :: thesis: verum

end;

then A4: cot is_differentiable_on Z by A3, FDIFF_1:9;
A5: Z c= dom sec by A2, XBOOLE_1:18;
A6: for x being Real st x in Z holds
( sec is_differentiable_in x & diff (sec,x) = (sin . x) / ((cos . x) ^2) )

proof

let x be Real; :: thesis: ( x in Z implies ( sec is_differentiable_in x & diff (sec,x) = (sin . x) / ((cos . x) ^2) ) )
assume x in Z ; :: thesis: ( sec is_differentiable_in x & diff (sec,x) = (sin . x) / ((cos . x) ^2) )
then cos . x <> 0 by A5, RFUNCT_1:3;
hence ( sec is_differentiable_in x & diff (sec,x) = (sin . x) / ((cos . x) ^2) ) by Th1; :: thesis: verum

end;

then for x being Real st x in Z holds
sec is_differentiable_in x ;
then A7: sec is_differentiable_on Z by A5, FDIFF_1:9;
for x being Real st x in Z holds
((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2))

proof

let x be Real; :: thesis: ( x in Z implies ((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) )
assume A8: x in Z ; :: thesis: ((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2))
then A9: sin . x <> 0 by A3, FDIFF_8:2;
((cot (#) sec) `| Z) . x = ((sec . x) * (diff (cot,x))) + ((cot . x) * (diff (sec,x))) by A1, A4, A7, A8, FDIFF_1:21
.= ((sec . x) * (- (1 / ((sin . x) ^2)))) + ((cot . x) * (diff (sec,x))) by A9, FDIFF_7:47
.= ((sec . x) * (- (1 / ((sin . x) ^2)))) + ((cot . x) * ((sin . x) / ((cos . x) ^2))) by A6, A8
.= ((- (1 / ((sin . x) ^2))) / (cos . x)) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) by A5, A8, RFUNCT_1:def 2 ;
hence ((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) ; :: thesis: verum

end;

hence ( cot (#) sec is_differentiable_on Z & ( for x being Real st x in Z holds
((cot (#) sec) `| Z) . x = (- ((1 / ((sin . x) ^2)) / (cos . x))) + (((cot . x) * (sin . x)) / ((cos . x) ^2)) ) ) by A1, A4, A7, FDIFF_1:21; :: thesis: verum