WAEC 2020 Mathematics · Question 30

The implication x →\to y is equivalent to?

Options

  1. A.~ y →\to ~ x
  2. B.y →\to ~ x
  3. C.~ x →\to ~ y
  4. D.y →\to x

Answer: A. ~ y →\to ~ x

Explanation written with AI and checked against the answer key

Analyze the question: We need the statement that is logically equivalent to the implication "x → y" (if x, then y).

Concept: Every implication is logically equivalent to its contrapositive. The contrapositive is formed by negating both statements and swapping their positions:

  • x → y ≡ ~y → ~x

The other related forms are not equivalent to the original:

  • Converse: y → x
  • Inverse: ~x → ~y (The converse and inverse are equivalent to each other, but not to the original.)

Truth table check:

| x | y | x → y | ~y → ~x | y → x | ~x → ~y | |---|---|-------|---------|-------|---------| | T | T | T | T | T | T | | T | F | F | F | T | T | | F | T | T | T | F | F | | F | F | T | T | T | T |

The column for x → y (T, F, T, T) matches ~y → ~x (T, F, T, T) exactly. It does not match the converse (y → x) or the inverse (~x → ~y).

Checking the options:

  • A. ~y → ~x is the contrapositive, so it is equivalent. ✔
  • B. y → ~x is not equivalent.
  • C. ~x → ~y is the inverse, so it is not equivalent.
  • D. y → x is the converse, so it is not equivalent.

Answer: A

Exam tip: Remember "flip and negate" for the contrapositive. If you flip only (converse) or negate only (inverse), the meaning changes. Only doing both keeps it equivalent.

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