Let A = \-3, -2, -1, 0, 1, 2, 3\ and R be a relation on A defined by xRy if and only if 2x - y \0, 1\. Let l be the number of elements in R. Let m and n be the minimum number of elements required to be added in R to make it reflexive and symmetric relations, respectively. Then l + m + n is equal to:
- 17
- 18
- 15
- 16
Answer
(1) 17
Solution
xRy y = 2x or y = 2x - 1, with x, y A.
x = -3: y = -6, -7 (not in A)
x = -2: y = -4, -5 (not in A)
x = -1: y = -2, -3 (-1, -2), (-1, -3)
x = 0: y = 0, -1 (0, 0), (0, -1)
x = 1: y = 2, 1 (1, 2), (1, 1)
x = 2: y = 4, 3 (2, 3)
x = 3: y = 6, 5 (not in A)
l = 7
Reflexive: all 7 pairs (a, a) are needed; only (0, 0) and (1, 1) are present m = 7 - 2 = 5
Symmetric: the reverses (-2, -1), (-3, -1), (-1, 0), (2, 1), (3, 2) are all missing n = 5
l + m + n = 7 + 5 + 5 = 17
Answer: (1)