Let A = \-3, -2, -1, 0, 1, 2, 3\. Let R be a relation on A defined by x R y if and only if 0 x2 + 2y 4. Let l be the number of elements in R and m be the minimum number of elements required to be added in R to make it a reflexive relation. Then l + m is equal to
- 18
- 20
- 17
- 19
Answer
(1) 18
Solution
0 x2 + 2y 4 ⇒ - x22 y 4 - x22
x = 3: -4.5 y -2.5 ⇒ y = -3 → 2 pairs
x = 2: -2 y 0 ⇒ y = -2, -1, 0 → 6 pairs
x = 1: -0.5 y 1.5 ⇒ y = 0, 1 → 4 pairs
x = 0: 0 y 2 ⇒ y = 0, 1, 2 → 3 pairs
∴ l = 2 + 6 + 4 + 3 = 15
Pairs (x, x) already in R: (-3, -3), (-2, -2), (0, 0), (1, 1)
Missing: (-1, -1), (2, 2), (3, 3) ⇒ m = 3
∴ l + m = 15 + 3 = 18
Answer: (1)