We're assuming f prime of a and g prime of a actually exist. But notice if we substitute a right over here, we get 0 over 0. But then if the derivatives exist, we can just evaluate the derivatives at a and then we get the limit. 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.