I am studying from Spivak' Calculus, and he states Taylor's Theorem as follows:
THEOREM
Let $f',\cdots,f^{(n+1)}$ be defined on $[a,x]$ and let $R_{n,a}(x)$ be defined by
$R_{n,a}(x)=f(x)-\sum_{k=0}^n \frac{f^{(k)}(a)}{k!}x^k$
Then, for some $t\in (a,x)$
$\eqalign{ & {R_{n,a}}(x) = \frac{{{f^{\left( {n + 1} \right)}}\left( t \right)}}{{n!}}{\left( {x - t} \right)^n}\left( {x - a} \right) \cr & {R_{n,a}}(x) = \frac{{{f^{\left( {n + 1} \right)}}\left( t \right)}}{{\left( {n + 1} \right)!}}{\left( {x - a} \right)^{n + 1}} \cr} $
Moreover, if $f^{(n+1)}$ is integrable over $[a,x]$; then
${R_{n,a}}(x) = \int\limits_a^x {\frac{{{f^{\left( {n + 1} \right)}}\left( t \right)}}{{n!}}{{\left( {x - t} \right)}^n}} dt$
On the other hand, Landau's older textbook states:
THEOREM Let $h>0$. Suppose $f^{(n)}$ continuous for $0\leq x\leq h$ and differentiable on $0
. Let $\Phi=f(h)-\sum_{k=0}^n \frac{f^{(k)}(0)}{k!}h^k$
Then, there exists a $t$ in $(0,h)$ such that
$\Phi=f^{(n+1)}(t)\frac{h^{n+1}}{(n+1)!}$
and then uses this to prove.
THEOREM (Taylor's Theorem) Let $h>0$. Suppose $f^{(n)}$ continuous for $\mu \leq x\leq \mu+h$ and differentiable on $\mu
. Then there exists an $t$ such that $\mu and$f(\mu+h)=\sum\limits_{v = 0}^n {\frac{{{f^{\left( v \right)}}\left( \mu \right)}}{{v!}}} {h^v} + \frac{{{h^{n+1}}}}{{(n+1)!}}{f^{\left( n+1 \right)}}\left( t \right)$
Now: I see the hypothesis are both the same, and they both address the remainder, but I can't see why Landau fixes $h>0$ and then gives a formula for this fixed $h$ and the $t$ (he actually names this $x$, but I found it a little conflicting) , while Spivak gives the remainder as a function of $x$ and a fixed $t$. Maybe it is just to make his (Landau's) proof simpler? I see how to go from Spivak's result to Landau's, but not the other way around.