General UNIX puzzle

[Brett Glass]

I was asked a good question today about how to do some simple tasks using 
standard UNIX tools, and am curious what answers people on the list might 
come up with.

What are the simplest, most efficient ways to:

1) Delete the Nth line from a text file?
2) Insert a specified line in a file after the Nth line, leaving the rest 
of the text the same?
3) Delete the first line containing a particular string?
4) Insert a specified line after the first line containing a particular 
string, leaving the rest of the file the same?

--Brett Glass