Let f : [0, 3] A be defined by f(x) = 2x3 - 15x2 + 36x + 7 and g : [0, ) B be defined by g(x) = x2025x2025 + 1. If both the functions are onto and S = \x Z; x A or x B\, then n(S) is equal to :
- 29
- 31
- 30
- 36
Answer
(3) 30
Solution
Since f and g are onto, A = range of f and B = range of g.
f'(x) = 6x2 - 30x + 36 = 6(x - 2)(x - 3)
f increases on [0, 2] and decreases on [2, 3].
f(0) = 7, f(2) = 16 - 60 + 72 + 7 = 35, f(3) = 54 - 135 + 108 + 7 = 34
A = [7, 35] integers 7, 8, …, 35: 29 integers
g(x) = 1 - 1x2025 + 1 increases from g(0) = 0 towards 1 (never reaching 1)
B = [0, 1) only integer 0
n(S) = 29 + 1 = 30
Answer: (3)