Help with Integral: \int \frac{e^t}{cos^2(5t)}sin(6t)dt

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Discussion Overview

The discussion revolves around the integral \(\int \frac{e^t}{\cos^2(5t)}\sin(6t)dt\), which has arisen in the context of solving a differential equation. Participants explore the possibility of finding an analytical solution and discuss various methods for tackling the integral.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about the integral being solvable analytically and mentions attempts at trigonometric manipulations and integration by parts without success.
  • Another participant asserts that a primitive of the function likely does not exist in closed form or elementary functions.
  • A third participant references Gradshteyn & Rhyzik, stating that integrals of the form involving exponential functions divided by trigonometric functions are not expressible as finite combinations of elementary functions, suggesting that this integral falls into a similar category.
  • This participant also proposes that an infinite series might be obtainable, or that the integral could be manageable if constant limits were applied.
  • The original poster acknowledges the difficulty in finding a closed form and considers using Taylor expansions to integrate the resulting series, noting the absence of constant limits due to the integral's context in an ordinary differential equation.

Areas of Agreement / Disagreement

Participants generally agree that the integral is unlikely to have a closed-form solution in elementary functions. However, there is no consensus on the best approach to handle the integral, with some suggesting series expansions while others highlight the limitations of the integral's form.

Contextual Notes

The discussion highlights the limitations of the integral's form and the dependency on the absence of constant limits, which may affect the methods available for finding a solution.

BobbyBear
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Hello, I need help witht he following integral which has come up while trying to solve a differential equation.

[tex] <br /> \int \frac{e^t}{cos^2(5t)}sin(6t)dt<br /> [/tex]

I'm not sure it can be integrated analytically, I 've tried to think of possible trigonometric manipulations and integrating by parts ... but not leading anywhere, any help would be appreciated.
 
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I am pretty sure a primitive of this function does not exist in closed form/elementary functions.
 
Gradshteyn & Rhyzik states that indefinite integrals of the forms exp[ax] divided by sin(bx), cos(bx), sin^2(bx) or cos^2(bx) aren't expressible as a finite combination of elementary functions, so your integral is similarly hopeless, as it's a linear combination of such integrals (for complex a).

You might be able to get an infinite series for it, though.

Or, if you had constant limits it might be a doable integral.
 
Thank you, I've been breaking my head trying to find a closed form in terms of elementary functions, but wasn't able to come up with anything. I guess the only way then is to use Taylor expansions or the like and then integrate the terms of the resulting series.

No, I don't have constant limits: the integral came up in the process of trying to find the solution function of an ordinary differential equation.

Thank you for your help :)
 

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