Why i see the following errors while updating in terminal?

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http://askubuntu.com – Fetched 1,103 kB in 1min 2s (17.6 kB/s) Reading package lists... Done W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://extras.ubuntu.com precise Release: The following signatures were invalid: BADSIG 16126D3A3E5C1192 Ubuntu Extras Archive Automatic Signing Key W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/precise/Release W: Some index files failed to download. They have been ignored, or old ones used instead. (HowTos)