# Role of mean value theorem in fundamental theorem of calculus proof

1. Jul 1, 2010

### dylanbyte

Hi, i've been watching the MIT lectures on single variable calculus, and whilst proving FTC, he mentions that we since we know that: <$> f'(x) = g'(x) </$>, then by MVT we know that <$> f(x) = g(x) + C </$>.

I have tried searching for somewhere where this implication is spelled out for me, but I'm having trouble.

As I understand MVT, it tells us that for two points on a function, there is a tangent, in that interval, parallel to the secant described by those points.

A quick explanation would be really appreciated so I can fully appreciate this goodly resource!

Thank you :)

2. Jul 1, 2010

### njama

f'(x) = g'(x) then by CDT (Constant Difference Theorem) f(x)=g(x)+C.

Here is something to know:

"The Constant Difference Theorem has a simple geometric interpretation - it tells us that if f and g have the same derivative on an interval, then there is a constant k such that f(x)=g(x)+k for each x in the interval; that is, the graphs of f and g can be obtained from one another by a vertical translation" - Calculus (Seventh Edition) by Howard Anton, Irl Bivens and Stephen Davis.

3. Jul 1, 2010

### snipez90

Analytically, the implication you are describing is a corollary of the fact that a function with derivative zero must be a constant. For a precise statement and the details, see http://en.wikipedia.org/wiki/Mean_value_theorem#A_simple_application". The corollary itself follows by defining a new function h to be the difference of f and g, i.e. h = f-g.

Last edited by a moderator: Apr 25, 2017
4. Jul 1, 2010

### Tedjn

This is how it follows directly from MVT: For all x, f'(x) = g'(x) means for all x, (f-g)'(x) = 0. Consider the function (f-g)(x). Since it has a derivative equal to 0, it is differentiable (hence also continuous) everywhere. Consider for x and y with x < y, (f-g)(x) and (f-g)(y). By the mean value theorem, in (x,y) there exists z such that [(f-g)(x) - (f-g)(y)]/(x-y) = (f-g)'(z) = 0. Hence, (f-g)(x) = (f-g)(y) = C.

EDIT: Well, I sort of jumped the gun on you, Office_Shredder -_-.

5. Jul 1, 2010

### Office_Shredder

Staff Emeritus
I assume the question is more about how the MVT is used for this statement than why it's true.

We can look at the function h(x)f(x)-g(x). It's derivative is zero, and I claim the MVT implies that means h(x) is constant. If you're not sure why, consider two points where h is unequal and ask what the MVT says about this situation

6. Jul 1, 2010

### dylanbyte

Ah thank you for the extremely prompt and enlightening replies, I am still learning and what seems obvious to most is rather opaque to me. My mistake here, was I think, taking MVT at its boring face value, rather than the practical information it gives us from the derivative be it neg, pos or 0. Although I guess they're the same thing.

Thanks again!