<script>
  function a1()
  {
    var a = document.getElementById("a");
    a.innerHTML = parseInt(a.innerHTML)+1;
  }
  function a2()
  {
    var a = document.getElementById("a");
    a.innerHTML = parseInt(a.innerHTML)-1;
  }
  function a3()
  {
    var a = document.getElementById("a");
    a.innerHTML = parseInt(a.innerHTML)+5;
  } 
  function a4()
  {
    var a = document.getElementById("a");
    a.innerHTML = parseInt(a.innerHTML)*parseInt(a.innerHTML);
  } 
  function a5()
  {
    var a = document.getElementById("a");
    a.innerHTML = Math.sqrt(parseInt(a.innerHTML));
  } 
  function f1()
  {
    var a = document.getElementById("a");
    var b = document.getElementById("textbox");
    a.innerHTML = b.value;
  } 
</script>
<button onclick="a1()">dodaj +1</button>
<button onclick="a2()">dodaj -1</button>
<button onclick="a3()">dodaj +5</button><br>
<button onclick="a4()">potega</button>
<button onclick="a5()">pierwiastek</button>
<hr>
<form>
  <input type="text" id="textbox">
  <input type="button" value="OK" onclick="f1()">
</form>
<hr>
<span id="a">0<span>