let h, x be Real; :: thesis: for f being Function of REAL,REAL holds [!f,(x - (h / 2)),(x + (h / 2))!] = ((cD (f,h)) . x) / h
let f be Function of REAL,REAL; :: thesis: [!f,(x - (h / 2)),(x + (h / 2))!] = ((cD (f,h)) . x) / h
[!f,(x - (h / 2)),(x + (h / 2))!] = [!f,(x + (h / 2)),(x - (h / 2))!] by DIFF_1:29
.= ((cD (f,h)) . x) / h by DIFF_1:5 ;
hence [!f,(x - (h / 2)),(x + (h / 2))!] = ((cD (f,h)) . x) / h ; :: thesis: verum