let Z be open Subset of REAL; :: thesis: ( Z c= dom (cosec * tan) implies ( - (cosec * tan) is_differentiable_on Z & ( for x being Real st x in Z holds
((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) ) ) )
assume A1: Z c= dom (cosec * tan) ; :: thesis: ( - (cosec * tan) is_differentiable_on Z & ( for x being Real st x in Z holds
((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) ) )
then A2: Z c= dom (- (cosec * tan)) by VALUED_1:8;
A3: cosec * tan is_differentiable_on Z by A1, FDIFF_9:40;
dom (cosec * tan) c= dom tan by RELAT_1:25;
then A4: Z c= dom tan by A1;
A5: (- 1) (#) (cosec * tan) is_differentiable_on Z by A2, A3, FDIFF_1:20;
A6: for x being Real st x in Z holds
sin . (tan . x) <> 0

proof

let x be Real; :: thesis: ( x in Z implies sin . (tan . x) <> 0 )
assume x in Z ; :: thesis: sin . (tan . x) <> 0
then tan . x in dom cosec by A1, FUNCT_1:11;
hence sin . (tan . x) <> 0 by RFUNCT_1:3; :: thesis: verum

end;

for x being Real st x in Z holds
((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2)

proof

let x be Real; :: thesis: ( x in Z implies ((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) )
assume A7: x in Z ; :: thesis: ((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2)
then A8: cos . x <> 0 by A4, FDIFF_8:1;
then A9: tan is_differentiable_in x by FDIFF_7:46;
A10: sin . (tan . x) <> 0 by A6, A7;
then A11: cosec is_differentiable_in tan . x by FDIFF_9:2;
A12: cosec * tan is_differentiable_in x by A3, A7, FDIFF_1:9;
((- (cosec * tan)) `| Z) . x = diff ((- (cosec * tan)),x) by A5, A7, FDIFF_1:def 7
.= (- 1) * (diff ((cosec * tan),x)) by A12, FDIFF_1:15
.= (- 1) * ((diff (cosec,(tan . x))) * (diff (tan,x))) by A9, A11, FDIFF_2:13
.= (- 1) * ((- ((cos . (tan . x)) / ((sin . (tan . x)) ^2))) * (diff (tan,x))) by A10, FDIFF_9:2
.= (- 1) * ((1 / ((cos . x) ^2)) * (- ((cos . (tan . x)) / ((sin . (tan . x)) ^2)))) by A8, FDIFF_7:46
.= ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) ;
hence ((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) ; :: thesis: verum

end;

hence ( - (cosec * tan) is_differentiable_on Z & ( for x being Real st x in Z holds
((- (cosec * tan)) `| Z) . x = ((cos . (tan . x)) / ((cos . x) ^2)) / ((sin . (tan . x)) ^2) ) ) by A5; :: thesis: verum