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11. Write out a proof of the following statement about integers. Use only the definitions of even and odd together with algebra. If y - xy is odd, then x is even and y is odd. 12. What are the two characteristics of a set? 13. How do you show A = B? 14. What are the two characteristics of a bag or multiset? 15. Describe the difference rule for counting sets. 16. For each integer n let An = {x | XEN and x sn). Evaluate each expression. a. A8 n A3 b. AS U A3 C. A8-A3 d. A3-A8