Integral involving the derivative

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    Derivative Integral
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Discussion Overview

The discussion centers around the calculation of the integral involving the derivative of a function, specifically the integral \(\int \frac{f'(x)}{f^{2}(x)}dx\). Participants explore different approaches to solving this integral, including rewriting it in terms of \(f\) and using substitution methods.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant suggests rewriting the integral as \(\int f^{-2}\,df\) and applying the power rule.
  • Another participant requests clarification on how the transformation from \(f'(x)\) and \(dx\) to \(df\) is achieved.
  • A later reply confirms the transformation and reiterates the equivalence of the two forms of the integral.
  • Some participants discuss the possibility of using the substitution rule, proposing \(u=f(x)\) and \(du=f'(x)\,dx\) to arrive at \(\int u^{-2}\,du\), while noting it may seem more complicated.

Areas of Agreement / Disagreement

Participants generally agree on the validity of the approaches discussed, but there is no consensus on the preferred method or whether one method is more straightforward than the other.

Contextual Notes

The discussion does not resolve the nuances of the transformation from \(dx\) to \(df\) or the implications of using different methods for solving the integral.

Yankel
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Hello,

I am trying to calculate the following integral:

\[\int \frac{f'(x)}{f^{2}(x)}dx\]

I suspect is has something to do with the rule of f'(x)/f(x), with the ln, but there must be more to it than that.

can you assist please ? Thank you !
 
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I would rewrite the given integral as:

$$\int f^{-2}\,df$$

Now just use the power rule. :)
 
Can you explain your solution please ? How did you go from the derivative and dx to f only with df ?
 
$$\int \frac{f'(x)}{f^2(x)}\,dx=\int f^{-2}\d{f}{x}\,dx=\int f^{-2}\,df$$
 
Right. Simple...

Could you get the same result from the substitution rule ?
 
Yankel said:
Right. Simple...

Could you get the same result from the substitution rule ?

Yes, I suppose we could write:

$$u=f(x)\implies du=f'(x)\,dx$$

And now we have:

$$\int u^{-2}\,du$$

That seems a little roundabout to me, but perfectly legitimate. :D
 

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