What is the correct formula for calculating focal length in convex mirrors?

[kankerfist]

I'm studying for a test that is going to cover mirrors and lenses. I was under the impression that the focal length for any concave or convex mirror was half the radius of curvature:
f=r/2

but one of the questions in my book involving a convex mirror uses a different value for f, according to the solutions manuel. It says that:
f=2/r

of all the questions I've encountered while studying for this test, f has always been r/2. can someone help explain why f can be 2/r?

 

[berkeman]

That does seem like a typo. As r gets bigger, f will grow. Can you state the exact problem from the text?

 

[kankerfist]

So was that a typo? Test tomorrow...

 

[OlderDan]

So was that a typo? Test tomorrow...

Yes it was a typo. They meant to say 1/f = 2/r and that is the way they used it in the problem. The right hand side of the next to last line replaced the 1/f with -2/20yd