Let A = \2, 3, 5, 7, 9\. Let R be the relation on A defined by xRy if and only if 2x 3y. 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 symmetric relation. Then l + m is equal to:
- 21
- 25
- 23
- 27
Answer
(2) 25
Solution
(x, y) R y 2x3
x = 2: y 43 y = 2, 3, 5, 7, 9 (5 pairs)
x = 3: y 2 y = 2, 3, 5, 7, 9 (5 pairs)
x = 5: y 103 y = 5, 7, 9 (3 pairs)
x = 7: y 143 y = 5, 7, 9 (3 pairs)
x = 9: y 6 y = 7, 9 (2 pairs)
l = 5 + 5 + 3 + 3 + 2 = 18
For symmetry, (y, x) must be added for every (x, y) R with (y, x) R:
(2, 5), (2, 7), (2, 9) R but (5, 2), (7, 2), (9, 2) R 3 pairs
(3, 5), (3, 7), (3, 9) R but (5, 3), (7, 3), (9, 3) R 3 pairs
(5, 9) R but (9, 5) R 1 pair
(Pairs among 5, 7, 9 other than these, and (2, 3), (3, 2), are already symmetric.)
m = 7
l + m = 18 + 7 = 25
Answer: (2)