Jo tak.

Třeba:

<form name="form1" action="zkoiuskajs.html">
<input type="text" name="vklad" id="vklad" value="" >
<input type="button" name="tlac" value="pocitej" onClick="areaOfCircle(document.getElementById('vklad').value);">
<div id="mainDiv">
</form>

<script type="text/javascript">

function areaOfCircle(radius) 
{
  Vysledek = Math.PI * Math.pow(radius, 2);
  document.getElementById("mainDiv").innerHTML += "<br><br>Výpočet " + Vysledek;
  
 }


</script>