máy tính bỏ túi bằng javascript

<html>
<script>
function tinh()
{
for(i=0;i<3;i++)
{
if(frm_dd.frm_a[i].checked == true)
{
alert('OK');
return true;
//break;
}
}
if(i==3)
alert('Ban phai chon');
return false;

}
function tinh1()
{
if(frm_ddd.frm_nam.checked==false && frm_ddd.frm_nu.checked==false && frm_ddd.frm_khac.checked==false)
{
alert('Ban phai tron 1');
return false;
}
else
{
alert("OK 1");
return true;
}
}
//************************************************** *****
function hienthi(a)
{
document.pheptinh.manhinh.value+=a;
}
function tinh()
{
document.pheptinh.manhinh.value=eval(document.phep tinh.manhinh.value);
}
function xoa()
{
document.pheptinh.manhinh.value="";
}
</script>

<body>
<form id="frm_dd" name="frm_dd" method="post" action="http://www.youtube.com/watch?v=Uquy4fImnLM">
<input name="frm_a" type="radio" value="Nam">
Nam
<input name="frm_a" type="radio" value="Nu">
Nu
<input name="frm_a" type="radio" value="Khac">
Khac
<input type="submit" name="Submit" value="OK" onClick="return tinh();">
<input type="reset" name="Submit2" value="Reset">
</form>

<form name="frm_ddd" id="frm_ddd">
<input name="frm_nam" type="checkbox" id="frm_nam" value="checkbox">
Nam
<input name="frm_nu" type="checkbox" id="frm_nu" value="checkbox">
Nu
<input name="frm_khac" type="checkbox" id="frm_khac" value="checkbox">
Khac
<input type="submit" name="Submit3" value="Ok" onClick="return tinh1();">
<input type="reset" name="Submit4" value="Reset">
</form>

<form name="pheptinh">
<table align="center">
<caption>khigiadano<br><input name="manhinh" type="text" size=15></caption>
<tr align="center" bgcolor="#00FFFF">
<td><input name"n_1" type="button" value="1" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_2" type="button" value="2" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_3" type="button" value="3" onClick="hienthi(this.value)" style="width:25px">
<td><input name"cong" type="button" value="+" onClick="hienthi(this.value)" style="width:25px">
<tr align="center">
<td><input name"n_4" type="button" value="4" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_5" type="button" value="5" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_6" type="button" value="6" onClick="hienthi(this.value)" style="width:25px">
<td><input name"tru" type="button" value="-" onClick="hienthi(this.value)" style="width:25px">
<tr align="center" bgcolor="#00FFFF">
<td><input name"n_7" type="button" value="7" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_8" type="button" value="8" onClick="hienthi(this.value)" style="width:25px">
<td><input name"n_9" type="button" value="9" onClick="hienthi(this.value)" style="width:25px">
<td><input name"nhan" type="button" value="*" onClick="hienthi(this.value)" style="width:25px">
<tr align="center">
<td><input name"c" type="button" value="C" onClick="xoa()" style="width:25px">
<td><input name"n_0" type="button" value="0" onClick="hienthi(this.value)" style="width:25px">
<td><input name"bang" type="button" value="=" onClick="tinh()" style="width:25px">
<td><input name"chia" type="button" value="/" onClick="hienthi(this.value)" style="width:25px">
</table>
</form>
</body>
</html>