Esempio minimo:
codice:
<script>
function x(v) {
	z = v * (v - 1) * 2;
	s = document.f.b;
	s.options.length = 0;
	for(var i=1;i<=z;i++) { 
		s.options[s.options.length] = new Option(i,i); 
	};
	s.selectedIndex = 0;
}
</script>
<body>
<form name="f">
<select name="a" onchange="x(this.options[this.selectedIndex].value)">
<script>
	for(var i=2;i<=10;i++) {
		document.write('<option value="'+i+'">'+i+'</option>')
	};
</script>
</select>
<select name="b">
</select>
<script>
	x(2);
</script>
</form>
</body>