M-ka,пока ненайдёте браузер в котором document.write выдадут разные значения, ваши слова бред.
<html>
<head>
<title>Untitled</title>
</head>
<body>
<table id="show_equipment">
<tr>
<td>1</td>
<td>1</td>
</tr>
<tr>
<td>2</td>
<td>2</td>
</tr>
<tr>
<td>3</td>
<td>3</td>
</tr>
<tr>
<td>4</td>
<td>4</td>
</tr>
<tr>
<td>5</td>
<td>5</td>
</tr>
</table>
<script>
document.write(document.getElementById('show_equipment').getElementsByTagName('tr').length);
var i = document.getElementById('show_equipment').getElementsByTagName('tr').length -1;
document.write(document.getElementById('show_equipment').getElementsByTagName('tr').length);
</script>
</body>
</html>