# nLab mean value theorem

The Mean Value Theorem

### Context

#### Differential geometry

synthetic differential geometry

Introductions

from point-set topology to differentiable manifolds

Differentials

V-manifolds

smooth space

Tangency

The magic algebraic facts

Theorems

Axiomatics

cohesion

tangent cohesion

differential cohesion

$\array{ && id &\dashv& id \\ && \vee && \vee \\ &\stackrel{fermionic}{}& \rightrightarrows &\dashv& \rightsquigarrow & \stackrel{bosonic}{} \\ && \bot && \bot \\ &\stackrel{bosonic}{} & \rightsquigarrow &\dashv& \mathrm{R}\!\!\mathrm{h} & \stackrel{rheonomic}{} \\ && \vee && \vee \\ &\stackrel{reduced}{} & \Re &\dashv& \Im & \stackrel{infinitesimal}{} \\ && \bot && \bot \\ &\stackrel{infinitesimal}{}& \Im &\dashv& \& & \stackrel{\text{étale}}{} \\ && \vee && \vee \\ &\stackrel{cohesive}{}& ʃ &\dashv& \flat & \stackrel{discrete}{} \\ && \bot && \bot \\ &\stackrel{discrete}{}& \flat &\dashv& \sharp & \stackrel{continuous}{} \\ && \vee && \vee \\ && \emptyset &\dashv& \ast }$

Models

Lie theory, ∞-Lie theory

differential equations, variational calculus

Chern-Weil theory, ∞-Chern-Weil theory

Cartan geometry (super, higher)

# The Mean Value Theorem

## Idea

The motivating idea of the derivative in differential calculus is that it is approximated by a ratio of differences. We may also say that an instantaneous rate of change is approximated by an average rate of change. The Mean Value Theorem reverses this, and says that any average rate of change is equal to some instantaneous rate of change, if certain differentiability conditions are met.

The name comes from the fact that, due to the fundamental theorem of calculus, an average rate of change over an interval may be viewed as an average (or mean) of the instantaneous rates of change along the interval. Thus, the theorem states that the mean value of the derivative on an interval is attained somewhere in that interval.

## Statements

There are traditionally three versions of increasing generality, although even the most general version is implicit in the most specific version (requiring only a linear coordinate transformation).

###### Rolle's Theorem

Suppose that $a \lt b$ are real numbers and $f$ is a continuous real-valued function on $[a,b]$. If $f$ is differentiable on the interior $]{a,b}[$, and if $f(a) = f(b)$, then for some $c \in {]{a,b}[}$,

$f'(c) = 0 .$
###### Lagrange's Theorem

Suppose that $a \lt b$ are real numbers and $f$ is a continuous real-valued function on $[a,b]$. If $f$ is differentiable on the interior $]{a,b}[$, then for some $c \in {]{a,b}[}$,

$f'(c) = \frac {f(b) - f(a)} {b - a} ,$

or equivalently

$f'(c) (b - a) = f(b) - f(a) .$
###### Cauchy's Theorem

Suppose that $a \lt b$ are real numbers and $f$ and $g$ are continuous real-valued functions on $[a,b]$. If $f$ and $g$ are differentiable on the interior $]{a,b}[$, then for some $c \in {]{a,b}[}$,

$f'(c) (g(b) - g(a)) = g'(c) (f(b) - f(a)) ;$

assuming that $f'$ and $g'$ are never simultaneously zero in $]{a,b}[$ and that $(f(a),g(a)) \neq (f(b),g(b))$, then for some $c \in {]{a,b}[}$,

$\frac {f'(c)} {g'(c)} = \frac {f(b) - f(a)} {g(b) - g(a)} ,$

where either side of this equation is allowed to be interpreted as $\infty$ in case it is division by zero (necessarily with a nonzero dividend under these conditions); or perhaps better, an equality of ratios:

$f'(c) : g'(c) :: f(b) - f(a) : g(b) - g(a) .$

If we write $u$ for $f(x)$ and $v$ for $g(x)$, then this last version states that

$\left.{\frac{\mathrm{d}u}{\mathrm{d}v}}\right|_{x=c} = \left.{\frac{\Delta{u}}{\Delta{v}}}\right|_{x=a}^b .$

Compare this to the definition

$\left.{\frac{\mathrm{d}u}{\mathrm{d}v}}\right|_{x=a} \coloneqq \lim_{b\to{a}} \left.{\frac{\Delta{u}}{\Delta{v}}}\right|_{x=a}^b$

(although this is really only a definition when $v$ is $x$, which reduces Cauchy's theorem to Lagrange's).

## References

Rolle's theorem is usually called just ‘Rolle's’ theorem, being the only result attributed today to Michel Rolle?; but Lagrange's and Cauchy's theorems must be called ‘mean value’ theorems, as Joseph-Louis Lagrange? and Augustin-Louis Cauchy did far more. By default, the term ‘Mean Value Theorem’ usually refers to Lagrange's theorem. (But neither Rolle nor Lagrange proved their theorem in the general case; the first proofs of all of them are due to Cauchy in 1823, a decade after Lagrange's death and more than a century after Rolle's death.)

Last revised on November 17, 2016 at 09:24:20. See the history of this page for a list of all contributions to it.