vawma,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
</head>
<body>
<div id="widget-a9681133-bdad-1cf5-39cc-3869f6131565" class="widget widget-text">
<div class="widget-content">
<h2 style="text-align: center;"> <span style="line-height: 1.42;"> </span><span style="font-size: 26px; line-height: 1.42;"> Найти значение "х"</span></h2>
<hr /></div>
</div>
<div id="widget-2a3eaccf-83e6-fe00-ceab-62e93f9fae90" class="widget widget-script">
<div class="widget-content"><center>
<div class="center"><form method="post" name="reqfocform">
<table class="calc" width="600">
<tbody>
<tr>
<td>a :</td>
<td><input type="text" name="a" value="" size="4" /> грамм</td>
</tr>
<tr>
<td>b :</td>
<td><input type="text" name="b" value="" size="4" /> грамм</td>
</tr>
<tr>
<td>c :</td>
<td><input type="text" name="c" value="" size="4" /> (Cu)</td>
</tr>
<tr>
<td class="result" colspan="3" height="35">x :<input type="text" name="x" value="" size="8" /> Гц</td>
</tr>
<tr>
<td class="calcbutton" rowspan="2"><input onclick="reqfoccalc()" type="button" value=" Подсчёт " /></td>
</tr>
</tbody>
</table>
<script>
function reqfoccalc()
{
var a = document.getElementsByName('a')[0].value|0,
b = document.getElementsByName('b')[0].value|0,
c = document.getElementsByName('c')[0].value|0,
x = document.getElementsByName('x')[0];
x.value = 159/(Math.sqrt(a+b+.5)*c)
}
</script>
</form></div>
</center></div>
</div>
</body>
</html>