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Predicates and Open Sentences

A free Logic lesson from the “Quantifiers and Predicates” unit, with a worked example and practice problems including step-by-step solutions.

A predicate is an open sentence with a variable. It becomes true or false only after the variable and domain are known. Learning objective: Recognize claims that become statements when a variable is given a value. Prerequisite: No formal prerequisite. Work in this lesson starts with ordinary language, then connects the idea to symbols only after the meaning is clear. Example 1: 'All integers are rational' is universal. Example 2: 'Some rectangles are squares' is existential because it claims at least one example. A common misconception is to treat familiar wording as proof; instead, check exactly what the statement says and what follows from it.

What you'll learn

Why it matters: All and some language appears in data claims, geometry theorems, function domains, and proof statements.

Worked example

Problem. Example case A (Predicates and Open Sentences): Worked example: Which rule or habit best matches Predicates and Open Sentences?

  1. Predicates and Open Sentences targets a specific reasoning habit.
  2. Predicates and Open Sentences focuses on recognize claims that become statements when a variable is given a value.
  3. The other choices either overclaim or change the logical relationship.

Answer: Predicates and Open Sentences focuses on recognize claims that become statements when a variable is given a value.

Practice problems

1. Practice case A (Predicates and Open Sentences): Which phrase signals a universal statement?

Choices: all · some · at least one · there exists

Show solution
  1. Warm-up: First identify exactly what the question is asking: Practice case A (Predicates and Open Sentences): Which phrase signals a universal statement?
  2. For quantified statements, identify the domain first, then decide whether the claim is about all objects or at least one object.
  3. Universal statements talk about every object in the domain.
  4. The word all signals that.
  5. Some and exists are existential.
  6. Verify the selected choice by checking that it preserves the stated logical meaning and that the other choices change the rule or claim.

Answer: all

2. Practice case B (Predicates and Open Sentences): Which phrase signals an existential statement?

Choices: at least one · every · all · no exceptions

Show solution
  1. Warm-up: First identify exactly what the question is asking: Practice case B (Predicates and Open Sentences): Which phrase signals an existential statement?
  2. For quantified statements, identify the domain first, then decide whether the claim is about all objects or at least one object.
  3. Existential statements claim an example exists.
  4. At least one means some object has the property.
  5. That is existential.
  6. Verify the selected choice by checking that it preserves the stated logical meaning and that the other choices change the rule or claim.

Answer: at least one

3. Practice case C (Predicates and Open Sentences): What is the negation of "All integers are positive"?

Choices: At least one integer is not positive. · No integers are positive. · All integers are negative. · Some integers are positive.

Show solution
  1. Warm-up: First identify exactly what the question is asking: Practice case C (Predicates and Open Sentences): What is the negation of "All integers are positive"?
  2. For quantified statements, identify the domain first, then decide whether the claim is about all objects or at least one object.
  3. The negation of all is at least one not.
  4. One counterexample makes the all claim false.
  5. No integers is too strong.
  6. Verify the selected choice by checking that it preserves the stated logical meaning and that the other choices change the rule or claim.

Answer: At least one integer is not positive.

4. Practice case D (Predicates and Open Sentences): What is the negation of "Some triangles are equilateral"?

Choices: No triangles are equilateral. · All triangles are equilateral. · Some triangles are not equilateral. · At least one triangle is equilateral.

Show solution
  1. Warm-up: First identify exactly what the question is asking: Practice case D (Predicates and Open Sentences): What is the negation of "Some triangles are equilateral"?
  2. For quantified statements, identify the domain first, then decide whether the claim is about all objects or at least one object.
  3. Some means at least one.
  4. The exact opposite is that none exist.
  5. So no triangles are equilateral.
  6. Verify the selected choice by checking that it preserves the stated logical meaning and that the other choices change the rule or claim.

Answer: No triangles are equilateral.

5. Practice case E (Predicates and Open Sentences): Lesson focus: Which transfer task would show readiness for Predicates and Open Sentences?

Choices: Apply recognize claims that become statements when a variable is given a value in a new sentence or context. · Repeat the exact worked example without changing the context. · Pick an answer before identifying the claim parts. · Use a rule from another lesson because the words sound close.

Show solution
  1. A transfer task keeps the same objective but changes the surface context.
  2. Apply recognize claims that become statements when a variable is given a value in a new sentence or context.
  3. That is a better mastery signal than memorizing a practice prompt.

Answer: Apply recognize claims that become statements when a variable is given a value in a new sentence or context.

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