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when ever I check for update ,while updating the cache this error occurs.

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 8771ADB0816950D8, W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)/dists/precise/main/binary-i386/Packages  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)/dists/precise/restricted/binary-i386/Packages  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, E:Some index files failed to download. They have been ignored, or old ones used instead.

please help me out..

thanks in advance

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Try with : sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com 8771ADB0816950D8 –  user224082 Dec 27 '13 at 5:02
    
i tried it but not working –  Jazzy Jan 1 at 16:48

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