reserve x,x1,x2,x3 for Real;

theorem Th2:
  sin(x)<>0 implies cos(x) = sin(x)*cot(x)
proof
  assume sin(x)<>0;
  then cos(x)= (sin(x)/sin(x))*cos(x) by XCMPLX_1:88
    .= sin(x)*cot(x) by XCMPLX_1:75;
  hence thesis;
end;
