# Fundamental Theorem of Calculus/First Part/Proof 1

## Theorem

Let $f$ be a real function which is continuous on the closed interval $\left[{a \,.\,.\, b}\right]$.

Let $F$ be a real function which is defined on $\left[{a \,.\,.\, b}\right]$ by:

$\displaystyle F \left({x}\right) = \int_a^x f \left({t}\right) \rd t$

Then $F$ is a primitive of $f$ on $\left[{a \,.\,.\, b}\right]$.

## Proof

To show that $F$ is a primitive of $f$ on $\left[{a \,.\,.\, b}\right]$, we need to establish the following:

• $F$ is continuous on $\left[{a \,.\,.\, b}\right]$
• $F$ is differentiable on the open interval $\left({a \,.\,.\, b}\right)$
• $\forall x \in \left[{a \,.\,.\, b}\right]: F^{\prime} \left({x}\right) = f \left({x}\right)$.

### Proof that $F$ is Continuous

We have that $f$ is continuous on $\left[{a \,.\,.\, b}\right]$.

It follows from Continuous Image of Closed Interval is Closed Interval that $f$ is bounded on $\left[{a \,.\,.\, b}\right]$.

Suppose that:

$\forall t \in \left[{a \,.\,.\, b}\right]: \left|{f \left({t}\right)}\right| < \kappa$

Let $x, \xi \in \left[{a \,.\,.\, b}\right]$.

From Sum of Integrals on Adjacent Intervals for Continuous Functions‎, we have that:

$\displaystyle \int_a^x f \left({t}\right) \rd t + \int_x^\xi f \left({t}\right) \rd t = \int_a^\xi f \left({t}\right) \rd t$

That is:

$\displaystyle F \left({x}\right) + \int_x^\xi f \left({t}\right) \rd t = F \left({\xi}\right)$

So:

$\displaystyle F \left({x}\right) - F \left({\xi}\right) = - \int_x^\xi f \left({t}\right) \rd t = \int_\xi^x f \left({t}\right) \ \rd t$
$\left|{F \left({x}\right) - F \left({\xi}\right)}\right| < \kappa \left|{x - \xi}\right|$

Thus it follows that $F$ is continuous on $\left[{a \,.\,.\, b}\right]$.

$\Box$

### Proof that $F$ is Differentiable and $f$ is its Derivative

It is now to be shown that that $F$ is differentiable on $\left({a \,.\,.\, b}\right)$ and that:

$\forall x \in \left[{a \,.\,.\, b}\right]: F' \left({x}\right) = f \left({x}\right)$

Let $x, \xi \in \left[{a \,.\,.\, b}\right]$ such that $x \ne \xi$.

Then:

 $\displaystyle \frac {F \left({x}\right) - F \left({\xi}\right)} {x - \xi} - f \left({\xi}\right)$ $=$ $\displaystyle \frac 1 {x - \xi} \left({F \left({x}\right) - F \left({\xi}\right) - \left({x - \xi}\right) f \left({\xi}\right)}\right)$ $\displaystyle$ $=$ $\displaystyle \frac 1 {x - \xi} \left({\int_\xi^x f \left({t}\right) \rd t - \left({x - \xi}\right) f \left({\xi}\right)}\right)$ $\displaystyle$ $=$ $\displaystyle \frac 1 {x - \xi} \int_\xi^x \left({f \left({t}\right) - f \left({\xi}\right)}\right) \rd t$ Integral of Function plus Constant, putting $c = f \left({\xi}\right)$

Now, let $\epsilon > 0$.

If $\xi \in \left({a \,.\,.\, b}\right)$, then $f$ is continuous at $\xi$.

So for some $\delta > 0$:

$\left|{f \left({t}\right) - f \left({\xi}\right)}\right| < \epsilon$

provided $\left|{t - \xi}\right| < \delta$.

So provided $\left|{x - \xi}\right| < \delta$ it follows that:

$\left|{f \left({t}\right) - f \left({\xi}\right)}\right| < \epsilon$

for any $t$ in an interval whose endpoints are $x$ and $\xi$.

So from the corollary to Upper and Lower Bounds of Integral, we have:

 $\displaystyle \left\vert{\frac {F \left({x}\right) - F \left({\xi}\right)} {x - \xi} - f \left({\xi}\right)}\right\vert$ $=$ $\displaystyle \frac 1 {\left\vert{x - \xi}\right\vert} \left\vert{\int_\xi^x \left({f \left({t}\right) - f \left({\xi}\right)}\right) \rd t}\right\vert$ $\displaystyle$ $<$ $\displaystyle \frac 1 {\left\vert{x - \xi}\right\vert} \epsilon \left\vert{x - \xi}\right\vert$ $\displaystyle$ $=$ $\displaystyle \epsilon$

provided $0 < \left|{x - \xi}\right| < \delta$.

But that's what this means:

$\dfrac {F \left({x}\right) - F \left({\xi}\right)} {x - \xi} \to f \left({\xi}\right)$ as $x \to \xi$

So $F$ is differentiable on $\left({a \,.\,.\, b}\right)$, and:

$\forall x \in \left[{a \,.\,.\, b}\right]: F' \left({x}\right) = f \left({x}\right)$

$\blacksquare$