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Logic MCQs — Mathematics (page 3)

64 multiple-choice questions on Logic from Mathematics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

51. An axiom is a statement that is:

  1. shown false by an example
  2. believed true but not yet proved
  3. proved using other theorems
  4. accepted as true without proof

Answer: D) accepted as true without proof

Explanation: Axioms (postulates) are the starting points of a mathematical system. SLO M-09-B-14 · Knowledge

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52. Pythagoras' result a^2 + b^2 = c^2 for right triangles is:

  1. an axiom
  2. a definition
  3. a conjecture
  4. a theorem

Answer: D) a theorem

Explanation: It has been proved in many ways. SLO M-09-B-14 · Knowledge

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53. The expression n^2 + n + 41 is prime for n = 0, 1, 2, ldots, 39. For n = 40 its value is 1681 = 41^2. This shows that:

  1. 40 cases make a proof
  2. 1681 is prime
  3. the expression is always prime
  4. a pattern can fail even after many cases, so it needs a proof

Answer: D) a pattern can fail even after many cases, so it needs a proof

Explanation: Forty examples worked, yet n = 40 is a counterexample. SLO M-09-B-13 · Knowledge

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54. The sum of an even number 2m and an odd number 2n + 1 is:

  1. 2(m + n), which is even
  2. 2(m + n) + 1, which is odd
  3. 2mn + 1
  4. m + n + 1

Answer: B) 2(m + n) + 1, which is odd

Explanation: 2m + 2n + 1 = 2(m + n) + 1. SLO M-09-B-15 · Knowledge

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55. A statement that has been proved is called a:

  1. theorem
  2. hypothesis
  3. counterexample
  4. conjecture

Answer: A) theorem

Explanation: Once proved, a statement becomes a theorem. SLO M-09-B-13 · Knowledge

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56. In geometry, the word postulate means the same as:

  1. conjecture
  2. axiom
  3. theorem
  4. lemma

Answer: B) axiom

Explanation: Postulates are axioms of geometry. SLO M-09-B-14 · Knowledge

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57. A corollary is:

  1. a definition
  2. an axiom
  3. a result that follows directly from a theorem
  4. an unproved statement

Answer: C) a result that follows directly from a theorem

Explanation: Example: from “angles of a triangle add to 180^circ” it follows that each angle of an equilateral triangle is 60^circ. SLO M-09-B-14 · Knowledge

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58. Which statement is an identity (true for every x)?

  1. x + 3 = 7
  2. (x + 3)^2 = x^2 + 3x + 9
  3. (x + 3)^2 = x^2 + 6x + 9
  4. (x + 3)^2 = x^2 + 9

Answer: C) (x + 3)^2 = x^2 + 6x + 9

Explanation: Only (x + 3)^2 = x^2 + 6x + 9 holds for all x. SLO M-09-B-15 · Knowledge

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59. The sum of four consecutive integers n, n + 1, n + 2, n + 3 equals:

  1. 4(n + 1)
  2. 4n + 6 = 2(2n + 3)
  3. 4n + 4
  4. 4n + 10

Answer: B) 4n + 6 = 2(2n + 3)

Explanation: So the sum is always even but never a multiple of 4. SLO M-09-B-15 · Knowledge

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60. A conjecture is a statement that:

  1. is believed to be true but has not been proved
  2. is known to be false
  3. has been proved
  4. is accepted without proof

Answer: A) is believed to be true but has not been proved

Explanation: A conjecture becomes a theorem once someone proves it. SLO M-09-B-14 · Knowledge

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61. The letters Q.E.D. at the end of a proof mean:

  1. which was to be demonstrated (the proof is complete)
  2. question ends directly
  3. quite easily done
  4. the next step follows

Answer: A) which was to be demonstrated (the proof is complete)

Explanation: Q.E.D. — quod erat demonstrandum. SLO M-09-B-13 · Knowledge

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62. A single example that shows a general statement is false is called a:

  1. counterexample
  2. proof
  3. theorem
  4. lemma

Answer: A) counterexample

Explanation: One counterexample is enough to disprove a general statement. SLO M-09-B-13 · Knowledge

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63. “The angles of a triangle add up to 180^circ” is:

  1. a conjecture
  2. a theorem
  3. an axiom
  4. a definition

Answer: B) a theorem

Explanation: It is proved using the properties of parallel lines. SLO M-09-B-14 · Knowledge

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64. Prove that the sum of three consecutive integers is divisible by 3. Writing them as n, n + 1, n + 2, the sum is:

  1. 3(n + 1)
  2. 3n + 2
  3. 3n
  4. n + 3

Answer: A) 3(n + 1)

Explanation: n + (n + 1) + (n + 2) = 3n + 3 = 3(n + 1), a multiple of 3. SLO M-09-B-15 · Knowledge

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