Let A = \0, 1, 2, 3, 4, 5\. Let R be a relation on A defined by (x, y) R if and only if \x, y\ \3, 4\. Then among the statements (S1): The number of elements in R is 18, and (S2): The relation R is symmetric but neither reflexive nor transitive
- both are false
- only (S1) is true
- only (S2) is true
- both are true
Answer
(3) only (S2) is true
Solution
For \x, y\ = 3, both x, y \0, 1, 2, 3\ with at least one of them equal to 3, so the number of pairs is 42 - 32 = 7.
For \x, y\ = 4, both x, y \0, 1, 2, 3, 4\ with at least one equal to 4, so the number of pairs is 52 - 42 = 9.
Hence n(R) = 7 + 9 = 16 18, so (S1) is false.
R is symmetric because \x, y\ = \y, x\, so (x, y) R implies (y, x) R.
It is not reflexive, since \0, 0\ = 0 \3, 4\ and so (0, 0) R.
It is not transitive, since (0, 3) R and (3, 0) R but (0, 0) R.
(S2) is true; only (S2) is true.
Answer: (3)