Solving Derivative Question: How to Explain Why it's Negative?

[gomes.]

Homework Statement

[PLAIN]http://img696.imageshack.us/img696/4462/124234c23c423.jpg

The Attempt at a Solution

I did the first part already, by letting
u = ax => dx = du/a, then just substituting in the values.

How do i explain why its negative?

 

[hgfalling]

Try comparing the value of $e^{-x}$ to the value of $(1+x)^{-1}$. Then prove the relationship you find, maybe with series expansions or some other way.

 

[LCKurtz]

Hint: Its value doesn't depend on α. Find a nice value of α. to try.

 

[gomes.]

Im still stuck :(

 

[LCKurtz]

What nice value of α have you tried?