Recall that a function f(x) is concave down at all points where its second derivative is negative; that is, f ′′(x) < 0.
From part (a), we know that
Using the quotient rule again, we find that
This simplifies to
The denominator is always positive (by the same reasoning as in the solution to part (b)), so f "(x) will be negative whenever the numerator is negative, in symbols,
To figure out for which values of x the inequality holds, factor the expression as
We do this because it's easier to figure out for what x each of , or is negative and we know that a product of three numbers is negative if one of them is negative and the other two are positive or if all three are negative. Therefore if or if is in the interval , in which case and is positive.