知道两点的坐标计算机出他们之间的距离

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function distance(form) {
var x1 = eval(form.x1.value);
var y1 = eval(form.y1.value);
var x2 = eval(form.x2.value);
var y2 = eval(form.y2.value);
var xdiff = x2 - x1;
var ydiff = y2 - y1;
form.answer.value = Math.pow((xdiff * xdiff + ydiff * ydiff), 0.5);
}
//  End -->
</script>
<center>
<form>
<table border=3 cellspacing=2 cellpadding=5>
<tr>
<td colspan=4 align=center>计算平面两点之间的距离</td>
</tr>
<tr>
<td colspan=2 align=center>A点</td>
<td colspan=2 align=center>B点</td>
</tr>
<tr>
<td align=center>A点横坐标</td>
<td align=center>A点纵坐标</td>
<td align=center>B点横坐标</td>
<td align=center>B点纵坐标</td>
</tr>
<tr>
<td align=center><input type=text name=x1 size=5></td>
<td align=center><input type=text name=y1 size=5></td>
<td align=center><input type=text name=x2 size=5></td>
<td align=center><input type=text name=y2 size=5></td>
</tr>
<tr>
<td colspan=4 align=center><input type=button value="计算距离" onClick="distance(this.form)"></td>
</tr>
<tr>
<td colspan=4 align=center><input type=text name=answer size=20></td>
</tr>
</table>
</form>
</center>