I've got a problem when I make sudo apt-get update. It was the error in my attached picture. I've read many solutions like my error. But I can't solve it.
My OS is Ubuntu 14.04.4 LTS.
W: GPG error: http://us.archive.ubuntu.com trusty Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
