Integration of this trigonometry function

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SUMMARY

The discussion centers on the integration of a trigonometric function involving the cosine function and polynomial terms. The user attempted to apply the substitution method using ##u=\cos x## and the identity ##\cos^2 (x) = \frac{1}{2} + \frac{1}{2} \cos (2x)## but was unsuccessful. The consensus is that a closed solution is unlikely due to the complexity of the function, particularly because the derivative of the trigonometric function does not align with the polynomial in ##x##. The suggestion was made that substituting ##x^2## for ##x## could yield a more manageable integral.

PREREQUISITES
  • Understanding of trigonometric identities, specifically ##\cos^2 (x)##.
  • Familiarity with integration techniques, including substitution methods.
  • Knowledge of polynomial functions and their derivatives.
  • Basic proficiency with mathematical software like WolframAlpha for solving integrals.
NEXT STEPS
  • Explore advanced integration techniques, particularly the Weierstraß substitution method.
  • Research the implications of substituting variables in integrals, focusing on trigonometric and polynomial combinations.
  • Study the properties of trigonometric functions and their derivatives in the context of integration.
  • Utilize WolframAlpha to analyze similar integrals and understand the limitations of closed-form solutions.
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Mathematicians, students studying calculus, and anyone interested in advanced integration techniques involving trigonometric and polynomial functions.

songoku
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Homework Statement
Find
$$\int x \sqrt{1+4 \cos^2 (x)} dx$$
Relevant Equations
High School Integration:
integration by substitution
integration by part
integration of trigonometry function
Is it possible to do the integration? That is the full question

I don't know where to start, try to use ##u=\cos x## and also ##\cos^2 (x) = \frac{1}{2} + \frac{1}{2} \cos (2x)## but failed.

Thanks
 
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WolframAlpha and I assume that there is no closed solution. The fact that ##x## occurs as polynomial and as trigonometric function (whose derivative is not the polynomial in ##x##) makes it impossible to use things like e.g. the Weierstraß substitution.
 
Last edited:
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Thank you very much fresh_42
 
Are you 100% sure you wrote the problem down right? If instead of x in the cosine you have an x2 you could make more progress. (How much progress would depend on what the new integral is)
 
Vanadium 50 said:
Are you 100% sure you wrote the problem down right? If instead of x in the cosine you have an x2 you could make more progress. (How much progress would depend on what the new integral is)
Yes 100% sure. I have re-checked several times when doing the question and before posting it here
 

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