Do it and write up a nice solution to be graded.

These are problems that you do not need to turn in, but may be helpful with other problems. It may be similar to another problem and have a solution in the book, to it may be an easier problem of the same general type, etc. You might want to look these over as you come to them, but if you are convinced that you could do the problem, just move on to the next one. The material from these problems will be covered in other problems.

This might be extra practice (good review for tests) or simply a problem that you should know how to do but which you do not have to write up for grading. You are responsible for the material in these problems, and the material may or may not be covered in other problems. One good way to use these is to read through them and ask yourself questions like the following: would I know how to do this problem if asked (outline the steps mentally without doing them) What difficulties could arise? How is this problem similar to or different from the graded problems near it?

This is truly optional. It may be an interesting problem, but you are not responsible for anything you would learn from that problem that is not already covered in other problems.