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Math 366 Practice Problems for Exam 2

1.Constructive Proof of an Existential Statement.
(a) Prove that there is an even integer n such that n mod 3 = 1.
(b) Prove that there exists a rational number q such that 9 q2 = 4.
(c) Prove that there exist two real numbers whose product is less than their sum.
(d) Prove that there exist two real numbers which are not equal to each other and whose product
is equal to their sum .
(e) Prove that there is an odd integer n such that n > 1 and n has the form 3k + 1 for some
integer k.

2. Direct Proof of a Universal Statement.
(a) Prove that if n is an integer which is divisible by 6 then n is divisible by 3.
(b) Prove that for any integers a, b, c, and d, if a divides b and c divides d then a · c divides b · d.
(c) Prove that the product of two odd integers is odd.
(d) Prove that for any sets A, B, and C, if A B and A C then A B ∩ C.
(e) Prove that if n is an integer which is divisible by 5 then 3n is divisible by 15.
(f) Prove that for any rational numbers a and b , if a ≠ 0 then there is a rational number x such
that ax + b = 0.
(g) Show that the reciprocal of any nonzero rational number is rational.

3. Proof by Cases.
(a) Prove that for every integer n, n and n + 2 have the same parity (i.e. either n and n + 2 are
both even or n and n + 2 are both odd).
(b) Prove that for any integer n, n2 + n is even.
(c) Prove that for any integers n and m, n2+3m ≠ 2. Hint: Use an argument by cases depending
on what the remainder is when n is divided by 3.
(d) Prove that for any sets A, B, and C, if A C then A ∪ (B ∩ C) C.

4. Mathematical Induction .
(a) Prove that for any integer n, if n ≥ 0 then 4 divides 5n - 1.
(b) Prove that for any integer n, if n ≥ 1 then 4 divides 6n - 2n.
(c) Show that 2n + 1 < 2n for every integer n with n ≥ 3.
(d) Using the fact that 2n+1 < 2n for every integer n with n ≥ 3, show that for every integer n,
if n ≥ 5 then n2 < 2n.

5. Strong Mathematical Induction and the Well- Ordering Principle .
(a) Suppose is a sequence defined as follows:



  for all integers k ≥ 2.

Prove that for all integers n ≥ 0.

(b) Suppose is a sequence defined as follows:



  for all integers k ≥ 2.

Prove that   for all integers n ≥ 0.

6. Proofs by Contradiction and Contraposition.
(a) Prove that there is no smallest real number x such that 1 < x < 2..
(b) For any integer n, if n2 is not divisible by 3 then n is not divisible by 3.

7. Computations with Sets.
(a) Let A = {a, c, d} and B = {b, c, f} be subsets of the universal set U = {a, b, c, d, e, f, g}.
Compute A ∪ B, A ∩ B, A − B, Ac, and A × B using “bracket” notation.
(b) Let A = {1, 3} and B = {2, 3} be subsets of the universal set U = {0, 1, 2, 3, 4}. Compute
A ∪ B, A ∩ B, A − B, Ac, and A × B using “bracket” notation.

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