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Homework Statement
Prove that [itex]0.493948<\int_0^{1/2} \frac{1}{1+x^4} dx<0.493958[/itex]
Homework Equations
This chapter is about Taylor Polynomials, and specifically this section deals with Taylor's formula with remainder:
[tex]f(x)=\sum_{k=0}^n \frac{f^{(k)}(a)}{k!} (x-a)^k + E_n(x)[/tex]
The general formula for [itex]E_n(x)[/itex] is given in integral form:
[tex]E_n(x)=\frac{1}{n!}\int_a^x (x-t)^n f^{(n+1)} (t) dt[/tex]
There is a theorem which gives bounds for the error:
If the (n+1)st derivative of f satisfies the inequalities
[tex]m \le f^{(n+1)} (t) \le M[/tex]
for all t in some interval containing a, then for every x in this interval we have the following estimates:
[tex]m \frac{(x-a)^{n+1}}{(n+1)!} \le E_n(x) \le M \frac{(x-a)^{n+1}}{(n+1)!} \qquad \text{if} \quad x>a[/tex]
(and a similar one if x<a, which I don't think we'll need since 1/2 > 0)
The Attempt at a Solution
The only example similar to this is approximating [itex]\int_0^{1/2} e^{-t^2}dt[/itex], which is much easier since we know that the nth derivative of [itex]e^x[/itex] is always [itex]e^x[/itex].
I've tried a number of methods, but all seem unlikely to be what Apostol had in mind, given their complexity. The most straightforward, I suppose, would be to use a method similar to the book's, however there's no telling what the n+1st derivative is in general, and to get close to the book's bounds we would need to get to at least the sixth derivative, and then figure out where it is equal to zero on (0,1/2) so that we could go back and put bounds on the fifth derivative (yuck!).
Alternatively, I had found that in the work for #3 (a lot of these problems build on previous ones) I had come up with the formula
[tex]\frac{1}{1+x^2} - \sum_{k=0}^{n-1}(-1)^k x^{2k} -x^{2n} \le 0[/tex]
Which, if we replace x by x^2, gives an integrable formula for the upper bound. There was also an analogous formula for the lower bound. The problem is, in order to approach the bounds given in the problem, I had to take it to n=4, which yields some serious arithmetic for a book written before calculators. Also, I don't get the lower bound exactly, I get a much higher lower bound.
Is this really what Apostol had in mind? I feel like I'm missing some crucial simplifying shortcut.
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