let h, x be Real; :: thesis: for f being Function of REAL,REAL st ( for x being Real holds f . x = (cot (#) sin) . x ) & x in dom cot & x - h in dom cot holds
(bD (f,h)) . x = (cos x) - (cos (x - h))
let f be Function of REAL,REAL; :: thesis: ( ( for x being Real holds f . x = (cot (#) sin) . x ) & x in dom cot & x - h in dom cot implies (bD (f,h)) . x = (cos x) - (cos (x - h)) )
assume that
A1: for x being Real holds f . x = (cot (#) sin) . x and
A2: ( x in dom cot & x - h in dom cot ) ; :: thesis: (bD (f,h)) . x = (cos x) - (cos (x - h))
(bD (f,h)) . x = (f . x) - (f . (x - h)) by DIFF_1:4
.= ((cot (#) sin) . x) - (f . (x - h)) by A1
.= ((cot (#) sin) . x) - ((cot (#) sin) . (x - h)) by A1
.= ((cot . x) * (sin . x)) - ((cot (#) sin) . (x - h)) by VALUED_1:5
.= ((cot . x) * (sin . x)) - ((cot . (x - h)) * (sin . (x - h))) by VALUED_1:5
.= (((cos . x) * ((sin . x) ")) * (sin . x)) - ((cot . (x - h)) * (sin . (x - h))) by A2, RFUNCT_1:def 1
.= (((cos x) / (sin x)) * (sin x)) - (((cos (x - h)) / (sin (x - h))) * (sin (x - h))) by A2, RFUNCT_1:def 1
.= ((cos x) / ((sin x) / (sin x))) - (((cos (x - h)) / (sin (x - h))) * (sin (x - h))) by XCMPLX_1:82
.= ((cos x) / ((sin x) * (1 / (sin x)))) - ((cos (x - h)) / ((sin (x - h)) / (sin (x - h)))) by XCMPLX_1:82
.= ((cos x) / 1) - ((cos (x - h)) / ((sin (x - h)) * (1 / (sin (x - h))))) by A2, FDIFF_8:2, XCMPLX_1:106
.= ((cos x) / 1) - ((cos (x - h)) / 1) by A2, FDIFF_8:2, XCMPLX_1:106
.= (cos x) - (cos (x - h)) ;
hence (bD (f,h)) . x = (cos x) - (cos (x - h)) ; :: thesis: verum