- #1
linearfish
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Homework Statement
Suppose that f is differentiable on the interval (a, inf) and f'(x) -> 0 as x -> inf. Show that f(x)/x -> 0 as x -> inf.
The Attempt at a Solution
Conceptually I understand this problem. The derivative gets very small so as x gets large, f(x) gets very close to being a constant. Meanwhile x grows without bound so f(x)/x -> 0. However I cannot seem to get the math to work. I've tried several approaches. This seems to be my best but I'm not positive of its validity.
Fix arbitrary epsilon > 0. Then there exists an M > 0 such that | f'(x) - 0 | = |f'(x)| < epsilon.
Choose c > M and let A = f(c). Then by the MVT there exists a k in (c, inf) such that.
[tex]\frac{f(x + c)-f(c)}{(x+c)-c} = \frac{f(x + c)-A}{x} = f'(k) < \epsilon[/tex]
Since A is fixed, then A/x -> 0 as x -> inf. This seems to suggest to me that we have shown the desired inequality but it bothers me I have f(x+c)/x instead of just f(x)/x. Can anyone nudge me in the right direction? Thanks.