一个新手问题

Ext.onReady(function(){
var rdaSexBoy = new Ext.form.Radio({
name:"rdaSex",
inputValue:"男",
boxLabel:"男",
checked:true
});
var rdaSexGirl = new Ext.form.Radio({
name:"rdaSex",
inputValue:"女",
boxLabel:"Girl"
});
var grpSex = new Ext.form.RadioGroup({
name: "sex",
fieldLabel: "性别",
items: [rdaSexBoy, rdaSexGirl],
width: 100
})
var panel = new Ext.form.FormPanel({
title:"What?",
width:300,
height:300,
items:[rdaSexBoy,rdaSexGirl,grpSex]

});
panel.render(Ext.getBody());
})
错误： Ext.form.RadioGroup is not a constructor