text: function tabuada(){
      var txtnum = document.getElementById('txtnum')
      var tab = document.getElementById('seltab')
      if(txtnum.value.length==0){
          window.alert('Numero Invalido!')
      }else{
          var num = Number(txtnum.value)
          var c = 1
          tab.innerHTML = ''
          while (c<=10){
              var item = document.createElement('option')
              //
              item.text = `${num} * ${c} = ${num*c}`
              item.value = `tab${c}`
              tab.appendChild(item)
              c++
          }
      }
      
  
  }
  