Let f: R (0, ) be a strictly increasing function such that x f(7x)f(x)=1. Then, the value of x [ f(5x)f(x)-1 ] is equal to
- 0
- 4
- 1
- 7/5
Answer
(1) 0
Solution
For x>0: x<5x<7x and f is increasing f(x)<f(5x)<f(7x)
∵ f(x)>0, divide by f(x): 1< f(5x)f(x)< f(7x)f(x)
As x the right side 1 ⇒ by Sandwich theorem f(5x)f(x) 1
x [ f(5x)f(x)-1 ]=0
Answer: (1)