function mostrar_letra(i){

    var visibilidad = document.getElementById("letra"+i).style.display;
//    var operacion = (visibilidad == "none") ? "show" : "hide";
//    $( "#letra"+i )[operacion]("blind",{},500,null);
    var operacion = (visibilidad == "none") ? "block" : "none";
    document.getElementById("letra"+i).style.display = operacion;
}