<form>
  <input type="text" value="0">
  <input type="text" value="0">
  <select onchange="calcul()">
    <option value="+">+</option>
    <option value="*">*</option>
  </select>
  <div id="rezultate">asdf</div>
</form>

<script>
  var op = document.getElementsByTagName('select')[0];
  op.addEventListener('change', calcul);


  function calcul(){
    var vals = document.getElementsByTagName('input');
    var val1 = parseInt(vals[0].value);
    var val2 = parseInt(vals[1].value);


    op = op.options.selectedIndex;

    var result = op===0 ? val1 + val2 : val1 * val2;
    var opr = op===0 ? '+' : '*';

    var resDiv = document.getElementById('rezultate');
    resDiv.innerHTML = val1 + opr + val2 +'='+result;
  }
</script>