Let f, g : (1, ) R be defined as f(x) = 2x + 35x + 2 and g(x) = 2 - 3x1 - x. If the range of the function f g : [2, 4] R is [ , ], then 1 - is equal to
- 56
- 2
- 29
- 68
Answer
(1) 56
Solution
g(x) = 2 - 3x1 - x = 3x - 2x - 1 = 3 + 1x - 1, which is decreasing on [2, 4].
g(2) = 4, g(4) = 3 + 13 = 103
for x [2, 4], g(x) [ 103, 4 ]
f(t) = 2t + 35t + 2 f'(t) = 2(5t + 2) - 5(2t + 3)(5t + 2)2 = -11(5t + 2)2 < 0, so f is decreasing.
f(4) = 1122 = 12 = 2856
f ( 103 ) = 203 + 3 503 + 2 = 2956
range of f g is [ 2856, 2956 ] = 2856, = 2956
- = 156 1 - = 56
Answer: (1)