If W's slope increases from that point, then this is an underestimate. If W's slope stays the same, then this is actually a very good estimate. Because then we're going to hit that point directly with W. And if W's slope decreases, so you can imagine maybe W looks something like this. It has that slope of the tangent line when it starts, but then the slope decreases. And it's still an increasing function, but the slope is decreasing. And in that case, we would have an overestimate. And this is a situation, so this first situation, slope is increasing, that means W prime prime is positive.