Selamat sore agan2, mohon pencerahannya.
Saya membuat membuat perhitungan rata2 pada tabel array, pada row pertama berhasil menghitung namun pada row berikutnya gagal. mohon bantuannya, terima kasih agan2...

https://img1.uploadhouse.com/fileuploads/26092/260925516e556090107abe660965bdcf9160283e.jpg

<script language="javascript" >
function OperasiPerhitungan(){
A1 = document.getElementById("angka1").value;
A2 = document.getElementById("angka2").value;
A3 = (parseFloat(A1) + parseFloat(A2))/2;
document.getElementById("hasil").value =A3;
}
</script>
	<div class="col-sm-9 col-sm-offset-3 col-lg-10 col-lg-offset-2 main">
		<div class="row">
			<div class="col-lg-12">
				<center><h3 class="page-header">EDIT NILAI</h3></center>
			</div>
		</div><!--/.row-->	
        <?php include ('koneksi.php');
        $nip=$_SESSION['$nip'];
        ?>
<div class="row">
			<div class="col-lg-12">
				<div class="panel panel-danger">
					<div class="panel-body">
					<form method="POST" action="input_nilai_edit_proses.php">
						<table id="example" class="" cellspacing="0" width="100%">
						        <thead>
						            <tr class="alert warning">
						            	<th>No</th>
						            	<th></th>
						            	<th>NIS</th>
						            	<th>Siswa</th>
						            	<th>UH 1</th>
						            	<th>UH 2</th>
								<th>RERATA</th>
						            </tr>
						        </thead>						
						        <tbody>
								<?php
								include('koneksi.php');
								$id_mapel = $_GET['id_mapel'];
								$query = mysql_query("SELECT siswa.nama,nilai.id_mapel, nilai.id_nilai, 
								nilai.nis, nilai.uh1,nilai.uh2
								FROM siswa, nilai WHERE siswa.nis=nilai.nis AND nilai.id_mapel= '$id_mapel'");
								 $no = 0;	//membuat variabel $no untuk membuat nomor urut
								while($data = mysql_fetch_array($query)){
								 $no++;
								 $id_nilai = $data['id_nilai'];
								$id_mapel = $data['id_mapel'];
								$rerata = $data['rerata'];
								?>
				<tr>
				<td><?php echo $no; ?></td>
				<td><input type="hidden" name="id_nilai[]" value="<?php echo $data['id_nilai']; ?>"></td>
				<td><?php echo $data['nis']; ?></td>
				<td><?php echo $data['nama']; ?></td>
				<td><input type="number" name="uh1[]" id="angka1" onchange="OperasiPerhitungan();" value="0" style="width:50px"></td>
				<td><input type="number" name="uh2[]" id="angka2" onchange="OperasiPerhitungan();" value="0" style="width:50px"></td>
				<td><input type="number" name="rerata[]" id="hasil" disabled="disabled" value="0" style="width:80px"></td>
				</tr>
						            <?php } ?>
						    	</tbody>
						    </table>
							<div class='navbar-form pull-left'>
							<button type="submit" name="tambah" class="btn btn-danger">Simpan</button>
							</div>
						</form> 

<tr>
                <td><?php echo $no; ?></td>
                <td><input type="hidden" name="id_nilai[]" value="<?php echo $data['id_nilai']; ?>"></td>
                <td><?php echo $data['nis']; ?></td>
                <td><?php echo $data['nama']; ?></td>
                <td><input type="number" name="uh1[]" id="angka1-<?php echo $no; ?>" onchange="OperasiPerhitungan(<?php echo $no; ?>);" value="0" style="width:50px"></td>
                <td><input type="number" name="uh2[]" id="angka2-<?php echo $no; ?>" onchange="OperasiPerhitungan(<?php echo $no; ?>);" value="0" style="width:50px"></td>
                <td><input type="number" name="rerata[]" id="hasil-<?php echo $no; ?>" disabled="disabled" value="0" style="width:80px"></td>
                </tr>

<script language="javascript" >
function OperasiPerhitungan(nomor){
    let A1 = document.getElementById("angka1-" + nomor).value;
    let A2 = document.getElementById("angka2-" + nomor).value;
    let A3 = (parseFloat(A1) + parseFloat(A2)) / 2;
    document.getElementById("hasil=" + nomor).value = A3;
}
</script>