So this is very close to the general case of L'Hospital's Rule. Now let's actually prove it. And to prove it, we're going to start with the right hand and then show that if we use the definition of derivatives, we get the left hand right over here. So let me do that. So I'll do it right over here. So f prime of a is equal to what by the definition of derivatives? Well, we could view that as the limit as x approaches a of f of x minus f of a over x minus a.