Ubuntu16.04更新软件源，执行apt-get update时出现“The following signatures couldn't be verified”错误

Ubuntu16.04中科大软件源：

deb http://mirrors.ustc.edu.cn/ubuntu/ trusty universe
deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty universe

Ubuntu16.04更新为中科大的软件源后，执行apt-get update时出现以下错误：

W: GPG error: https://mirrors.ustc.edu.cn trusty InRelease: 
              The following signatures couldn't be verified 
              because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8

解决方法：

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7EA0A9C3F273FCD8

再次执行：

sudo apt-get update