If 5f(x)+4f ( 1x )=x2-2, x 0 and y=9x2f(x), then y is strictly increasing in :
- (0, 1 5 ) ( 1 5, )
- (- 1 5, 0 ) ( 1 5, )
- (- 1 5, 0 ) (0, 1 5 )
- (- , 1 5 ) (0, 1 5 )
Answer
(2) (- 1 5, 0 ) ( 1 5, )
Solution
5f(x)+4f ( 1x )=x2-2 ...(1)
x 1x: 5f ( 1x )+4f(x)= 1x2-2 ...(2)
5 (1)-4 (2): 9f(x)=5x2- 4x2-2
y=9x2f(x)=5x4-2x2-4
dydx=20x3-4x=4x(5x2-1)>0
[figure]
∴ x (- 1 5, 0 ) ( 1 5, )
Answer: (2)