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The Mean Value Theorem

After arriving at our Aunt Petunia's house for Thansgiving, we figured out that our average speed for the trip was 60 mph. Before diving into the sweet potato casserole, though, we asked ourselves a very important question. Does this mean that there must have been a point in the trip when we were driving exactly 60 mph?

Think of this way. Our speed increases continuously; we can't just jump from 30 to 40 mph without hitting every speed in between. This leaves us with a few options:

Maybe we drove 60 mph for the whole trip? In this case, there definitely was a time we were driving exactly 60 mph.

Another option is that we were driving less than 60 mph during the trip. We couldn't have been doing this for the whole trip, though, since our average speed was 60 mph. This means that at some point we would have had to speed up and go more than 60 mph for everything to balance out. When we were accelerating, we must have hit 60 mph at some point.

The last option is that we were driving more than 60 mph during the trip. Like before, we couldn't have been doing this for the whole trip, otherwise our average speed would have been more than 60 mph. In order for everything to balance out, we would have needed to slow down to a speed less than 60 mph, and pass 60 mph along the way.

The scenario we just described is an intuitive explanation of the Mean Value Theorem.

The Mean Value Theorem is a glorified version of Rolle's Theorem.

More formally, The Mean Value Theorem states:   

If f is continuous on [a, b], and f is differentiable on (a, b), then there is some c in (a, b) with 

 is the slope of the secant line between the points (af(a)) and (bf(b)), or the "average speed" of the function between these points if we want to think of it that way.

As with the Intermediate Value Theorem and Rolle's Theorem, the conclusion of the Mean Value Theorem leaves a lot of things out. It doesn't tell us the value of c where f ' (c) equals the slope of the secant line. It doesn't tell us how many such values of c there are. All the Mean Value Theorem does is guarantee that if all the hypotheses are met, at least one such c exists.

As with all the other theorems, if the necessary hypotheses aren't met we aren't allowed to use the Mean Value Theorem.

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