What is the intuitive meaning of the Mean Value Theorem?

In summary, the mean value theorem states that if a function is continuous on a closed interval and has a derivative in the open interval, then there exists at least one point within the interval where the derivative is equal to the average rate of change between the endpoints. This theorem is important in understanding the concept of anti-derivatives and finding the rate of change at a specific point. It also has real-world applications, such as calculating average speeds or finding points of equal velocity in a race.
  • #1
semidevil
157
2
so by the defn:

suppose that f is continuous on a closed I:= [a,b] and that f has a derivative in the open interval (a,b). then tehre exists at least one point c in (a,b) st f(b) - f(a) = f'(c)(b - a).


ok, so what if I put this in terms of f'(c)? isn't that the definition of the derivative?

So it's saying that if f has a derivative in (a,b), then there is a point c that has a derivative?

I"m kind of lost...because this sounds a bit redundant.

I"m kind of having trouble on what the mean value theorem is telling me...
 
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  • #2
Yes that's another way to define the derivative. The Theorem is saying that if f(x) has a derivative in (a,b), then there is a number c in (a,b) such that is derivative is equal to what you saw.

If you look up Rolle Theorem you might understand more the importance of the mean value theorem.
 
  • #3
Starting with the equation:

f(b) - f(a) = f'(c)(b - a)

and dividing by (b - a) gives

f'(c) = [f(b)-f(a)]/(b - a)

Can you see what that fraction on the right hand side is?
 
  • #4
semidevil said:
so by the defn:

suppose that f is continuous on a closed I:= [a,b] and that f has a derivative in the open interval (a,b). then tehre exists at least one point c in (a,b) st f(b) - f(a) = f'(c)(b - a).


ok, so what if I put this in terms of f'(c)? isn't that the definition of the derivative?

So it's saying that if f has a derivative in (a,b), then there is a point c that has a derivative?

I"m kind of lost...because this sounds a bit redundant.

I"m kind of having trouble on what the mean value theorem is telling me...

No, that's NOT the definition of the derivative: (f(b)-f(a))/(b-a) is the slope of the straight line from (a,f(a)) to (b,f(b)) a secant line rather than a tangent line. The derivative requires a limit.

Geometrically what the mean value theorem says is that if f(x) is differentiable between a and b and continuous (from the left and right) at a and b, then there exist some point between a and b where the tangent line is parallel to the the straight line between a and b. The reason for the name "mean value" theorem is that (f(b)-f(a))/(b-a) is a sort of "average" ("mean") value of the derivative at all points between a and b- and the mean value theorem says that there is some point where the actual derivative is the same as the average derivative.

The mean value theorem is crucial in dealing with "anti-derivatives". You know that many functions have the same derivative: f(x)= 4, g(x)= 1000 both have the same derivative: 0; f(x)= x2+ 10 and g(x)= x2+ 100000 both have the same derivative: 2x. Of course, they differ by a constant and the derivative of a constant is 0 (from the basic definition). But how do we know there are not other, perhaps extremely complicated functions that also have that derivative? How do we know that there are not other, perhaps extremely complicated functions that have derivative 0 for all x? From the mean-value theorem. If f'(x)= 0 for all x, then for any a, b,
f(b)-f(a)= f'(c)(b-a)= 0(b-a)= 0. In other words, f(a)= f(b) for any a and b: f is a constant.

As a reminder that f(x) continuous at the endpoints is necessary, consider
f(x)= 2x if x< 3, f(x)= 100 if x>= 3. f is differentiable between 0 and 3 and its derivative there is 2. If f(3)- f(0)= 2(3-0)?
 
  • #5
semidevil said:
I"m kind of having trouble on what the mean value theorem is telling me...

Drawing a picture may help. If you draw a straight line from (a,f(a)) to (b,f(b)), then the mean value theorem says that there is a point in (a,b) such that its tangent is parallel to that line.
In other words: There is some point at which the rate of change equals the average change.
Intuitively this is clear. It says for example that if a car has traveled 180 km in 2 hours, there must have been a point where it went 90 km/h.

Or, if two runners have a race and start and finish at the same time, then there must've been a point where their speed where equal.
 

1. What is the mean value theorem?

The mean value theorem is a fundamental theorem in calculus that states that for a differentiable function on a closed interval, there exists at least one point on that interval where the slope of the tangent line is equal to the average rate of change of the function over that interval.

2. How is the mean value theorem used in calculus?

The mean value theorem is used in calculus to prove many other important theorems, such as the fundamental theorem of calculus and Rolle's theorem. It is also used to find the maximum and minimum values of a function and to approximate the value of a definite integral.

3. What are the conditions for the mean value theorem to hold?

The mean value theorem holds if the function is continuous on a closed interval and differentiable on the open interval within that closed interval. In other words, the function must be continuous and have a derivative at every point within the interval.

4. Can the mean value theorem be applied to all functions?

No, the mean value theorem can only be applied to differentiable functions. If a function is not differentiable, then the theorem does not hold and cannot be used.

5. What is the significance of the mean value theorem?

The mean value theorem is significant because it provides a powerful tool for proving other theorems and solving problems in calculus. It also helps to understand the relationship between the derivative and the original function, as well as the behavior of the function on a given interval.

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