A1: for x being Real st x in ].PI,(2 * PI).[ holds
diff (cos,x) > 0

proof

let x be Real; :: thesis: ( x in ].PI,(2 * PI).[ implies diff (cos,x) > 0 )
assume x in ].PI,(2 * PI).[ ; :: thesis: diff (cos,x) > 0
then 0 - (sin . x) > 0 by Th25, XREAL_1:52;
hence diff (cos,x) > 0 by SIN_COS:72; :: thesis: verum

end;

].PI,(2 * PI).[ is open by RCOMP_1:25;
hence cos | ].PI,(2 * PI).[ is increasing by A1, FDIFF_1:34, ROLLE:9, SIN_COS:27, SIN_COS:72; :: thesis: verum