Integration by Parts with Domain Warning

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Discussion Overview

The discussion revolves around the integration of the function $\int x\ \cot^2\left({x}\right) dx$ using integration by parts. Participants explore the choice of variables for integration, the computation of the integral, and the implications of domain warnings encountered during the process.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes using $u=x$ and $dv=\cot^2\left({x}\right) dx$ for integration by parts, leading to $du=dx$.
  • Another participant requests clarification on the computation of $v$, suggesting that $v$ should be derived from integrating $\cot^2(x)$ as $v=\int (\csc^2 x -1) \,dx$.
  • A third participant agrees with the choice of $u$ and $dv$, but points out a potential error in the differentiation and asks for the value of $v$.
  • Some participants express uncertainty about the result for $v$, with one mentioning a domain warning returned by their calculator.
  • There is a suggestion that the calculator may have produced a specific expression for $v$, which includes terms involving $\cos(x)$ and $\sin(x)$.
  • Participants discuss the expression $uv-\int v\ du$ and the resulting terms, indicating a complex integration process.

Areas of Agreement / Disagreement

Participants generally agree on the initial choice of $u$ and $dv$, but there is disagreement and uncertainty regarding the computation of $v$ and the implications of the domain warning. The discussion remains unresolved regarding the correct form of $v$ and the overall integration process.

Contextual Notes

There are limitations in the discussion regarding the assumptions made in the integration process, the dependence on the correctness of the calculator outputs, and the potential impact of domain issues on the integration results.

karush
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$\int x\ \cot^2\left({x}\right) dx $
$u=x$ $dv=\cot^2\left({x}\right) dx $
$du=\frac{x^2}{2}$ $v=\frac{-\cos\left({x}\right)+x\sin\left({x}\right)}{\sin\left({x}\right)}$
 
Last edited:
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Hi karush,

Could you show your workings for $v$?
We have $dv=\cot^2\left({x}\right) dx$
$v=\int (\csc^2 x -1) \,dx$

Can you proceed?
 
I agree with you choice of $u$ and $dv$:

$$u=x\,\therefore\,du=dx$$ (You integrated rather than differentiated)

$$dv=\cot^2(x)\,dx=\left(\csc^2(x)-1\right)\,dx\,\therefore\,v=?$$
 
That what I got for $v$ with the TI ??
 
karush said:
That what I got for $v$ with the TI ??

I suspect your TI spat out:

$$v=-\frac{\cos\left({x}\right)+x\sin\left({x}\right)}{\sin\left({x}\right)}$$ :D
 
$uv-\int\ v\ du$

$-x(\cot\left({x}\right)-x)+\int \cot\left({x}\right)dx -\int x\ dx$

My TI returned a domain warning??
 

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