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21\. For the following statements the universe comprises all nonzero integers. Determine the truth value of each statement. a) \(\exists x \exists y[x y=1]\) b) \(\exists x \forall y[x y=1]\) c) \(\forall x \exists y[x y=1]\) d) \(\exists x \exists y[(2 x+y=5) \wedge(x-3 y=-8)]\) e) \(\exists x \exists y[(3 x-y=7) \wedge(2 x+4 y=3)]\)

Short Answer

Expert verified
The truth values of the given logical statements are: a) True b) False c) True d) True e) False

Step by step solution

01

Evaluate Statement a

Statement a says, 'there exist numbers x and y such that their product is 1'. This is true. For instance, if x and y are both 1, their product is indeed 1.
02

Evaluate Statement b

Statement b says, 'there exists a number x, for which all numbers y when multiplied with x give 1'. This is false. The only number which when multiplied with all other numbers gives 1 is 1 itself. But for integers other than 1, the condition fails, hence the statement is false.
03

Evaluate Statement c

Statement c states 'for every x, there exists a y such that their product is 1'. This is true because for every non-zero integer x, its reciprocal (1/x) which is the y in this case, gives the product 1.
04

Evaluate Statement d

Statement d is a system of linear equations with 2 variables x and y. Using substitution or elimination method, we find that there is a solution for x and y which satisfies both equations. Therefore, this statement is true.
05

Evaluate Statement e

Statement e is also a system of linear equations with 2 variables x and y. Solving the equations, we find that there is no pair of x, y that can satisfy both equations at the same time. Therefore, this statement is false.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Understanding Quantifiers in Discrete Mathematics
Quantifiers are symbols used in logic and mathematics to express the extent to which a statement is true, often dealing with sets or large quantities of elements. They help us specify constraints or guidelines for statements involving a range of items. In logic, the two main types of quantifiers are existential and universal.

  • **Existential Quantifier (\(\exists\))**: Indicates there is at least one element in the universe that satisfies a particular condition. For example, \(\exists x\) means 'there exists an \(x\)' such that a particular proposition is true.
  • **Universal Quantifier (\(\forall\))**: Suggests that a specified property holds for all elements in the considered set. For example, \(\forall x\) implies 'for all \(x\)' in a given set, a specified condition holds true.
When evaluating expressions with quantifiers, one must consider the domain of discourse, which contains all possible values that variables can assume. In the exercise provided, the universe is all nonzero integers. This means that any element considered under these quantifiers must be a nonzero integer.

By understanding and applying these quantifiers effectively, we can derive precise logical conclusions from statements, tackling various problems from simple assertions to complex mathematical proofs.
Systems of Linear Equations in Discrete Math
A system of linear equations involves two or more linear equations containing multiple variables. The ultimate goal is to find values for these variables that satisfy all equations simultaneously.

The diagrammatic representation is often a set of lines in a multi-dimensional space (like the x-y coordinate plane for 2 variables). Each equation represents one line, and solutions are the points where these lines intersect.

To solve such systems, we use methods like:
  • **Substitution**: Solve one equation for one variable and substitute that into another equation.
  • **Elimination**: Add or subtract equations to eliminate a variable, making it easier to solve.
The solution to a system can fall into three categories:
  • **One Solution**: Lines intersect at a single point.
  • **No Solution**: Lines are parallel and never intersect.
  • **Infinitely Many Solutions**: Lines coincide, meaning they lie on top of each other fully overlapping.
Understanding these systems' solutions is crucial for applications in discrete math, as they provide insights into solving real-world problems that utilize constraints and relationships among variables.
Explaining Truth Values in Logic
Truth values are fundamental components in logic, where they assess whether statements are true or false. This binary approach enables clear decision-making in logical operations. The truth value of a statement is crucial for evaluating logical expressions, especially when quantifiers are involved.

Logical expressions determine truth values based on the validity of their components. For example, if a statement using a universal quantifier, like \( \forall x [P(x)] \), is true, it means every possible instance of \( x \) makes \( P(x) \) true. If even one instance makes \( P(x) \) false, the whole statement is false.

For existential quantifiers, such as \( \exists x [P(x)] \), if at least one value of \( x \) makes \( P(x) \) true, then the statement holds true. Otherwise, it is false. In the context of our exercise, determining the truth value involves checking if specific conditions exist within the given universe.

Ultimately, grasping truth values empowers us to interpret logical arguments accurately and solve mathematical problems by confirming the validity of proposed statements.

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Most popular questions from this chapter

6\. Let \(p(x, y), q(x, y)\) denote the following open statements. $$ p(x, y): \quad x^{2} \geq y \quad q(x, y): \quad x+2

5\. Professor Carlson's class in mechanics is comprised of 29 students of which exactly 1) three physics majors are juniors; 2) two electrical engineering majors are juniors; 3) four mathematics majors are juniors; 4) twelve physics majors are seniors, 5) four electrical engineering majors are seniors; 6) two electrical engineering majors are graduate students; and 7) two mathematics majors are graduate students. Consider the following open statements. \(c(x):\) Student \(x\) is in the class (that is, Professor Carlson's mechanics class as already described). \(j(x): \quad\) Student \(x\) is a junior. \(s(x): \quad\) Student \(x\) is a senior. \(g(x)\) : Student \(x\) is a graduate student. \(p(x): \quad\) Student \(x\) is a physics major. \(e(x): \quad\) Student \(x\) is an electrical engineering major. \(m(x):\) Student \(x\) is a mathematics major. Write each of the following statements in terms of quantifiers and the open statements \(c(x), j(x), s(x), g(x), p(x), e(x)\), and \(m(x)\), and determine whether the given statement is true or false. Here the universe comprises all of the 12,500 students enrolled at the university where Professor Carlson teaches. Furthermore, at this university each student has only one major. a) There is a mathematics major in the class who is a junior. b) There is a senior in the class who is not a mathematics major. c) Every student in the class is majoring in mathematics or physics. d) No graduate student in the class is a physics major. e) Every senior in the class is majoring in either physics or electrical engineering.

14\. Prove that for every integer \(n\), if \(n\) is odd, then \(n^{2}\) is odd.

5\. Negate and express each of the following statements in smooth English. a) Kelsey will get a good education if she puts her studies before her interest in cheerleading. b) Norma is doing her homework, and Karen is practicing her piano lessons. c) If Harold passes his C++ course and finishes his data structures project, then he will graduate at the end of the semester.

12\. Write the following argument in symbolic form. Then either establish the validity of the argument or provide a counterexample to show that it is invalid. If it is cool this Friday, then Craig will wear his suede jacket if the pockets are mended. The forecast for Friday calls for cool weather, but the pockets have not been mended. Therefore Craig won't be wearing his suede jacket this Friday.

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