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The Honor Code of Stanford University and the rules for the CS 103X: Discrete Structures Midterm Exam held on February 8, 2007. The Honor Code outlines the expectations of students and faculty regarding academic integrity. The exam rules include the time limit, materials allowed, and instructions for completing the exam. The exam also includes exercises related to sets, rational and irrational numbers, and 3-dimensional space. The document could be useful as study notes, summaries, or exam preparation for students taking a course on discrete structures or related topics.
Typology: Exams
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Exams are to be done individually and must represent original workāit is a violation of the honor code to copy or derive exam question solutions from other students or anyone at all, textbooks, or previous instances of this course.
I acknowledge and accept the honor code:
Signature:
Name (print):
Exercise 1 (20 points). Prove or disprove: For any three sets A, B, C,
(a) C \ (A ā© B) = (C \ A) ā© (C \ B).
(b) C āŖ (A ā© B) = (C āŖ A) ā© (C āŖ B).
Exercise 2 (20 points). You have been appointed Postmaster General for a new nation. Your job is to determine what denominations to issue stamps in. Unfortunately, the government wants to be able to charge an amount for each letter or package that corresponds exactly to its weight, so the postage fee could be any integral number of cents. The government is also cheap and wants to reduce costs by only printing a minimal number of types of stamps, and stamps of value less than 5 cents will not be allowed.
(a) What is the minimum number n of integer values v 1 , v 2 ,... , vn (where vi ā„ 5 for all i) such that all positive integers can be expressed as linear combinations of them? What is the precise condition that such a minimal set v 1 , v 2 ,... , vn has to satisfy? Prove. Does your answer apply to real life?
(b) Suppose the government caps the lowest possible package charge at c cents, for some c > 50. In a āreal lifeā scenario, what is now the minimum number of stamp values v 1 , v 2 ,... (where vi ā„ 5 for all i) that you can get away with so as to cover all package weights greater or equal to c? Prove.
Exercise 5 (20 points). Consider n planes in 3-dimensional space so that no two are parallel, any three have exactly one point in common, and no four have a common point. What is the number of 3-dimensional parts into which these planes partition the space? Prove. (You can use the 2-dimensional result without proof.)