theorem Th28:
  f1 is_partial_differentiable_in x,i & f2
is_partial_differentiable_in x,i implies f1+f2 is_partial_differentiable_in x,i
  & partdiff(f1+f2,x,i)=partdiff(f1,x,i)+partdiff(f2,x,i)
proof
  assume that
A1: f1 is_partial_differentiable_in x,i and
A2: f2 is_partial_differentiable_in x,i;
A3: f1*reproj(i,x) is_differentiable_in Proj(i,m).x by A1;
A4: f2*reproj(i,x) is_differentiable_in Proj(i,m).x by A2;
  (f1+f2)*reproj(i,x) = f1*reproj(i,x)+f2*reproj(i,x) by Th26;
  then (f1+f2)*reproj(i,x) is_differentiable_in Proj(i,m).x by A3,A4,NDIFF_1:35
;
  hence f1+f2 is_partial_differentiable_in x,i;
  diff(f1*reproj(i,x)+f2*reproj(i,x),Proj(i,m).x) = partdiff((f1+f2),x,i)
  by Th26;
  hence thesis by A3,A4,NDIFF_1:35;
end;
