Linux：Unable to locate package错误解决

sudo apt-get install apachec2，结果出现了下面的Unable to locate package错误：

root@ubuntu:~# apt-get install apachec2
Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package apachec2

　　问题出在执行sudo apt-get install之前更换了软件源，但没有update，执行下面的命令：

sudo apt-get update
等上面命令执行完后，再执行sudo apt-get install