Let R be a relation defined on the set \1, 2, 3, 4\ \1, 2, 3, 4\ by R = \((a, b), (c, d)) : 2a + 3b = 3c + 4d\. Then the number of elements in R is
- 6
- 15
- 12
- 18
Answer
(3) 12
Solution
2a + 3b = 3c + 4d 2(a - 2d) = 3(c - b)
c - b = 2t and a - 2d = 3t for an integer t; with a, b, c, d \1, 2, 3, 4\, |c - b| 3 t \-1, 0, 1\
t = 0: c = b (4 ways), a = 2d (a, d) = (2, 1), (4, 2) (2 ways) 4 2 = 8
t = 1: a = 2d + 3 5, impossible 0
t = -1: c = b - 2 b \3, 4\ (2 ways); a = 2d - 3 (a, d) = (1, 2), (3, 3) (2 ways) 2 2 = 4
number of elements in R = 8 + 0 + 4 = 12
Answer: (3)