<script>
document.all.div2.style.display='none';
</script>
<form name = form1>
<div id =div1>
<input type= radio name=r1 id =a1 value =a onclick ="">a
</div>
<input type =radio name =r1 id =b1 value =b onclick="document.all.div1.style.display = 'none';document.all.div2.style.display='';">b
<div id = div2>
<input type = text>
</div>
</form>