theorem
  z = <*x0,y0*> & f is_partial_differentiable_in z,1 implies ex N being
  Neighbourhood of x0 st N c= dom SVF1(1,f,z) & ex L,R st for x st x in N holds
  SVF1(1,f,z).x - SVF1(1,f,z).x0 = L.(x-x0) + R.(x-x0)
proof
  assume that
A1: z = <*x0,y0*> and
A2: f is_partial_differentiable_in z,1;
  ex x1,y1 st z = <*x1,y1*> & SVF1(1,f,z) is_differentiable_in x1 by A2,Th5;
  then SVF1(1,f,z) is_differentiable_in x0 by A1,FINSEQ_1:77;
  hence thesis by FDIFF_1:def 4;
end;
