Should I acknowledge the source of a problem in a math assignment? For a math assignment that I am giving my students, I have included a problem from Spivak Calculus. I have the following two issues and I do not know what I should do:


If I do not acknowledge the source, it seems that I have committed plagiarism. 
And if I acknowledge the source, then the students will go and perhaps find the solution manual and give the same solution as in the solution manual.


Can anyone please discuss what is the best thing to do in my case?
I often assign "challenge" problems for my Calculus class that are drawn from Apostol (similar in difficulty, etc., to Spivak).  For the reasons you describe above, I prefer not to reveal the source of the problem.  My solution has been to label the problem as a challenge problem, and say that it comes from a classic (read: hard) textbook on Calculus when I assign the problem. However, I don't specify which textbook that is on the assignment.  After the problem set is due, I give the students the full reference.  Of course, my hope is that some of them found the problem interesting and will go look up more problems to challenge themselves.

I think this compromise works since you are not taking credit for creation of the problem, and do give acknowledgement.  Of course, with all of these students can find the answers through diligent searching, but I think raising the barrier even a little is a good idea.