Substitutions for some trig integrals?

In summary, the conversation discusses substitutions that can be used for three integrals: \int (a^2 - b^2 \cos\theta)^{-3/2} d\theta, \int (a^2 - b^2 \cos\theta)^{-3/2} \sin\theta d\theta, and \int (a^2 - b^2 \cos\theta)^{-3/2} \cos\theta d\theta. The individual is unsure of what substitutions to use and has looked in their table of integrals but did not find anything suitable. They then realize how to approach the second integral and that it leads to the desired form for the second two integrals. However, they
  • #1
Peeter
305
3
Does anybody know substitutions that can be used for the following integrals:

[tex]
\int (a^2 - b^2 \cos\theta)^{-3/2} d\theta
[/tex]
[tex]
\int (a^2 - b^2 \cos\theta)^{-3/2} \sin\theta d\theta
[/tex]
[tex]
\int (a^2 - b^2 \cos\theta)^{-3/2} \cos\theta d\theta
[/tex]

I also had a look for these in my table of integrals, but didn't see anything appropriate, but am also a bit rusty, and could just be overlooking the obvious.

EDIT: oops. fixed sign in square root.
 
Last edited:
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  • #2
Funny, how asking the question is enough to see the answer. I see what to do with the second one now, and it gets me halfway to:

[tex]
\int (a^2 - b^2 \cos\theta)^{-3/2} e^{i\theta} d\theta
[/tex]

which is actually what I was after for the second two. However, it's still not obvious to me what to do with the first nor third integrals.
 

FAQ: Substitutions for some trig integrals?

What is a substitution in trigonometric integrals?

A substitution in trigonometric integrals is a method used to simplify and evaluate integrals that involve trigonometric functions. It involves replacing a trigonometric expression with a new variable, making the integral easier to solve.

How do I know when to use a substitution in a trigonometric integral?

You can use a substitution in a trigonometric integral when you have an integral that involves a trigonometric function and its derivative. The new variable should be chosen in such a way that it simplifies the integral.

What are some common substitutions used in trigonometric integrals?

Some common substitutions used in trigonometric integrals include: u = sin(x), u = cos(x), u = tan(x), and u = sec(x). These substitutions are often used when the integral involves trigonometric functions and their derivatives.

Can I use any substitution in a trigonometric integral?

No, not all substitutions can be used in trigonometric integrals. The substitution should be chosen in such a way that it simplifies the integral and does not make it more complicated. It is important to carefully choose the substitution to ensure that the integral can be evaluated.

Are there any tips for choosing the right substitution in a trigonometric integral?

One tip for choosing the right substitution in a trigonometric integral is to look for patterns and try to match the integral to a known form. It is also helpful to practice and familiarize yourself with different substitutions and their applications in trigonometric integrals.

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