The following signatures couldn't be verified because the public key is not available

ubuntu在更换第三方软件源后，执行sudo apt-get update出现以下问题：

W: GPG error: http://ppa.launchpad.net precise 

 Release: The following signatures couldn't be verified because the public key is not available: 
 NO_PUBKEY 2EA8F35793D8809A

解决方法，缺少哪些key使用下面方法安装就行：

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2EA8F35793D8809A