theorem Th86:
  f is_differentiable_in x & f.x > -1 & f.x < 1 implies (arccot)*f
  is_differentiable_in x & diff((arccot)*f,x) = -diff(f,x)/(1+(f.x)^2)
proof
  assume that
A1: f is_differentiable_in x and
A2: f.x > -1 and
A3: f.x < 1;
  f.x in ]. -1,1 .[ by A2,A3,XXREAL_1:4;
  then
A4: arccot is_differentiable_in f.x by Th74,FDIFF_1:9;
  then diff(arccot*f,x) = diff(arccot,f.x)*diff(f,x) by A1,FDIFF_2:13
    .= diff(f,x)*(-1/(1+(f.x)^2)) by A2,A3,Th76
    .= -diff(f,x)/(1+(f.x)^2);
  hence thesis by A1,A4,FDIFF_2:13;
end;
