how get id="cb<?= $i?>" ?

<script class="jsbin" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

<script type="text/javascript">

$(document).ready(
function() {
$('#cb').change(
function() {
var amount = $('#principal_amount').val()*$('#principal_installment').val();
$('#amount').val(amount);
}
);
}
);

</script>
<tr>
<td><input type="checkbox" name="id[]" value="<?= $row2["id_principal_payment"]?>" id="cb<?= $i?>" onclick="isChecked(this.checked);" />
</td>

Thx,