apt-get update报错 Failed to fetch bzip2 Some index files failed to download

apt-get update时报错：
W: Failed to fetch bzip2:
/var/lib/apt/lists/partial/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.
出错原因：软件更新源过期或者无效
解决办法：
参考网页：http://jingyan.baidu.com/article/afd8f4de66efcf34e286e993.html?st=2&os=0&bd_page_type=1&net_type=2

问题解决！！！