I've just been using the numerator of the second derivative. The whole second derivative is this thing right here, but I've been ignoring the denominator, because the denominator is always positive. If we're trying to understand whether things are positive or negative, we just really have to determine whether the numerator is positive or negative, because this expression right here is always positive. It's something to the second power. Let's test whether we have a change in concavity around x is equal to positive or negative 3. Remember, the numerator of our... Let me just rewrite our second derivative, just so you see it here. f prime prime of x.