Appendix 1: Proofs in Mathematics | A1.2

Question 1

Given that all women are mortal, and suppose that A is a woman, what can we conclude about A?

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Solution
Understand the Question
  • We are given two logical statements:
    • Major premise (general rule): All women are mortal.
    • Minor premise (specific fact): A is a woman.
  • In deductive reasoning, if a general rule applies to an entire group and an individual belongs to that group, the rule must also apply to that individual.
  • Therefore, we can deduce a definite conclusion about A.

Step 1 · Identify the Given Premises

We are given two premises:

  1. General statement: All women are mortal.Diagram 1

  2. Specific fact: A is a woman.Diagram 2

Step 2 · Deduce the Conclusion

Since A belongs to the group of women, and all women are mortal, it logically follows that:

A is mortal\text{A is mortal}

Answer

A is mortal.

Common Mistakes
  • Assuming the Converse: Incorrectly assuming that being mortal means someone is necessarily a woman. Deductive logic works from the whole group to the subset, not in reverse.
  • Inductive vs. Deductive Confusion: Treating the deduction as an uncertain guess or generalization rather than a logically guaranteed fact based on the given premises.

More questions in A1.2

Q1

Given that all women are mortal, and suppose that A is a woman, what can we conclude about A?

Q2

Given that the product of two rational numbers is rational, and suppose aa and bb are rationals, what can you conclude about abab?

Q3

Given that the decimal expansion of irrational numbers is non-terminating, non-recurring, and 17\sqrt{17} is irrational, what can we conclude about the decimal expansion of 17\sqrt{17}?

Q4

Given that y=x2+6y = x^2 + 6 and x=1x = -1, what can we conclude about the value of yy?

Q5

Given that ABCD is a parallelogram and B=80\angle B = 80^\circ. What can you conclude about the other angles of the parallelogram?

Q6

Given that PQRSPQRS is a cyclic quadrilateral and also its diagonals bisect each other. What can you conclude about the quadrilateral?

Q7

Given that p\sqrt{p} is irrational for all primes pp and also suppose that 3721 is a prime. Can you conclude that 3721\sqrt{3721} is an irrational number? Is your conclusion correct? Why or why not?

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