# Math 201-B Iowa State University Introduction to Proofs Department of Mathematics Math 201-B Introduction to Proofs Instructor: Alex Roitershtein

### Exam #2 November 13, 2015

Iowa State University Department of Mathematics Fall 2015 Student name: - Duration of the exam 50 minutes.

- The exam includes 5 regular questions and a bonus problem.

- The total mark is 100 points for regular questions.

- Calculators, textbooks, and help sheets are allowed.

Student ID: 1.

[20 points] (a) Prove that if A, B, and C are sets such that A ∩ ( B − C ) = ∅ and then A ∩ ( C ( B ∪ C ) − ( B ∩ C ) ⊆ A.

− B ) = ∅ , (b) Prove that Z = { 2 m + 3 n : m, n ∈ Z } .

2.

[20 points] Prove or disprove: (a) A × B = C × D if and only if A = C and B = D.

(b) ( x, y ) ∈ R 2 : | x | + | y | ≤ 1 ⊆ ( x, y ) ∈ R 2 : x 2 + y 2 ≤ 1 .

3.

[20 points] Using induction , show that ( n 3 + 2 n ) is a multiplyer of 3 for all n ∈ N .

4.

[20 points] (a) Let R be a relation on Z such that xRy if either x ≡ y (mod 3) or else x ≡ y (mod 5) .

Is R reflexive? Symmetric? Transitive?

(b) Let R be a relation on Z such that xRy if 2 x + 5 y ≡ 0 (mod 3) .

Is R an equivalence relation?

5.

[20 points] Exercise 12 from Section 12.2 of the textbook BP.

6.

[Bonus] Is it possible to alternate just exactly one symbol/letter in the statement of Problem 4(b) in such a way that the answer will change to the opposite one?

1