let Z be open Subset of REAL ; :: thesis: ( Z c= dom (sec * cot ) implies ( sec * cot is_differentiable_on Z & ( for x being Real st x in Z holds
((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) ) ) )
assume A1: Z c= dom (sec * cot ) ; :: thesis: ( sec * cot is_differentiable_on Z & ( for x being Real st x in Z holds
((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) ) )
dom (sec * cot ) c= dom cot by RELAT_1:44;
then A2: Z c= dom cot by A1, XBOOLE_1:1;
A3: for x being Real st x in Z holds
cos . (cot . x) <> 0

proof

let x be Real; :: thesis: ( x in Z implies cos . (cot . x) <> 0 )
assume x in Z ; :: thesis: cos . (cot . x) <> 0
then cot . x in dom sec by A1, FUNCT_1:21;
hence cos . (cot . x) <> 0 by RFUNCT_1:13; :: thesis: verum

end;

A4: for x being Real st x in Z holds
sec * cot is_differentiable_in x

proof

let x be Real; :: thesis: ( x in Z implies sec * cot is_differentiable_in x )
assume A5: x in Z ; :: thesis: sec * cot is_differentiable_in x
then sin . x <> 0 by A2, FDIFF_8:2;
then A6: cot is_differentiable_in x by FDIFF_7:47;
cos . (cot . x) <> 0 by A3, A5;
then sec is_differentiable_in cot . x by Th1;
hence sec * cot is_differentiable_in x by A6, FDIFF_2:13; :: thesis: verum

end;

then A7: sec * cot is_differentiable_on Z by A1, FDIFF_1:16;
for x being Real st x in Z holds
((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 ))

proof

let x be Real; :: thesis: ( x in Z implies ((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) )
assume A8: x in Z ; :: thesis: ((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 ))
then A9: sin . x <> 0 by A2, FDIFF_8:2;
then A10: cot is_differentiable_in x by FDIFF_7:47;
A11: cos . (cot . x) <> 0 by A3, A8;
then sec is_differentiable_in cot . x by Th1;
then diff (sec * cot ),x = (diff sec ,(cot . x)) * (diff cot ,x) by A10, FDIFF_2:13
.= ((sin . (cot . x)) / ((cos . (cot . x)) ^2 )) * (diff cot ,x) by A11, Th1
.= (- (1 / ((sin . x) ^2 ))) * ((sin . (cot . x)) / ((cos . (cot . x)) ^2 )) by A9, FDIFF_7:47
.= - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) ;
hence ((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) by A7, A8, FDIFF_1:def 8; :: thesis: verum

end;

hence ( sec * cot is_differentiable_on Z & ( for x being Real st x in Z holds
((sec * cot ) `| Z) . x = - (((sin . (cot . x)) / ((sin . x) ^2 )) / ((cos . (cot . x)) ^2 )) ) ) by A1, A4, FDIFF_1:16; :: thesis: verum