Why Ignore the Integration Constant in ∫xcos(x) dx?

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Discussion Overview

The discussion revolves around the treatment of the integration constant when performing integration by parts on the integral of xcos(x) dx. Participants explore whether the constant should be considered during intermediate steps and its implications on the final result.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question why the integration constant C is often ignored during intermediate steps in integration by parts.
  • Others suggest that it is acceptable to ignore the constant in intermediate steps, as it does not affect the final result, but acknowledge that the final integration could differ if the constant is not zero.
  • A participant explains that the integration constant cancels out in the integration by parts formula, leading to the same result regardless of its inclusion.
  • Another participant proposes an alternative approach by letting u = x and dv = cos(x), suggesting that this choice may be more effective, especially for integrals involving powers of x.
  • There is mention of using Tabular Integration By Parts as a potentially better method for this integral.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of the integration constant during intermediate steps, with no consensus reached on whether it should be included or ignored.

Contextual Notes

The discussion highlights the dependence on the choice of variables in integration by parts and the implications of ignoring constants, but does not resolve the mathematical nuances involved.

MHD93
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Hi

When we use the integration by parts to find the integral of xcosx dx, we assume that cosx dx = du and integrate both sides to find u
When we integrate
du = cosxdx
we find that u = sinx
the question is why don't we take the integration constant C into account?
 
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Mohammad_93 said:
the question is why don't we take the integration constant C into account?

You can take it into account if you want, you'll get the same result (except for a final integration constant of course), so it's usually better to assume this constant zero in the intermediate steps.
 
so it's usually better to assume this constant zero in the intermediate steps.

But it may or mayn't be zero, therefore the final integration is different if it's not zero.
 
I indeed am in need of your help
 
The point is that it cancels out. Without the integration constant:
[tex]\int{udv} = uv-\int{vdu}[/tex]
With the integration constant:
[tex]\intudv = u(v+C)-\int{(v+C)du} = uv+uC-\int{vdu}-C\int{du} = uv +uC-\int{vdu}-uC = uv-\int{vdu}[/tex]
 
Wow, that's real helpful, thank you
 
Mohammad_93 said:
Hi

When we use the integration by parts to find the integral of xcosx dx, we assume that cosx dx = du and integrate both sides to find u When we integrate du = cosxdx we find that u = sinx

For this integrand it is much better to let u = x and dv = Cos x
Your choice will work too, but if x is raised to a power the clear choice is
u = x^n and dv = Cos x
This integral is also best done with Tabular Integration By Parts
 

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