let x0 be Real; :: thesis: for I being Function of REAL,(REAL-NS 1) st I = (proj (1,1)) " holds
( I is_differentiable_in x0 & diff (I,x0) = <*1*> )
let I be Function of REAL,(REAL-NS 1); :: thesis: ( I = (proj (1,1)) " implies ( I is_differentiable_in x0 & diff (I,x0) = <*1*> ) )
assume A1: I = (proj (1,1)) " ; :: thesis: ( I is_differentiable_in x0 & diff (I,x0) = <*1*> )
I . 1 = <*1*> by A1, PDIFF_1:1;
then reconsider r = <*1*> as Point of (REAL-NS 1) ;
A2: for x being Real st x in [#] REAL holds
I /. x = (x * r) + (0. (REAL-NS 1))

proof

let x be Real; :: thesis: ( x in [#] REAL implies I /. x = (x * r) + (0. (REAL-NS 1)) )
assume x in [#] REAL ; :: thesis: I /. x = (x * r) + (0. (REAL-NS 1))
I . 1 in REAL 1 by A1, FUNCT_1:3, PDIFF_1:2;
then A3: <*1*> is Element of REAL 1 by A1, PDIFF_1:1;
I /. x = <*(x * 1)*> by A1, PDIFF_1:1
.= x * <*1*> by RVSUM_1:47 ;
hence I /. x = x * r by A3, REAL_NS1:3
.= (x * r) + (0. (REAL-NS 1)) by RLVECT_1:4 ;
:: thesis: verum

end;

A4: [#] REAL c= dom I by FUNCT_2:def 1;
then A5: ( I is_differentiable_on [#] REAL & ( for x being Real st x in [#] REAL holds
(I `| ([#] REAL)) . x = r ) ) by A2, NDIFF_3:21;
r = (I `| ([#] REAL)) . x0 by A4, A2, NDIFF_3:21
.= diff (I,x0) by A5, NDIFF_3:def 6 ;
hence ( I is_differentiable_in x0 & diff (I,x0) = <*1*> ) by A5, NDIFF_3:10; :: thesis: verum