reserve X for set;
reserve k,m,n for Nat;
reserve i for Integer;
reserve a,b,c,d,e,g,p,r,x,y for Real;
reserve z for Complex;

theorem Th15:
  tan r = (cot r)" & cot r = (tan r)"
  proof
    thus tan r = (sin r*(cos r)")" "
    .= ((sin r)"*(cos r)" ")" by XCMPLX_1:204
    .= (cot r)";
    thus cot r = (cos r*(sin r)")" "
    .= ((cos r)"*(sin r)" ")" by XCMPLX_1:204
    .= (tan r)";
  end;
