How to Use Trig Substitution for Integrals Involving (x²-a²)

In summary, the conversation discusses different methods for solving the integral ∫ dx/(x² -a² ), including using trig substitution and partial fractions. The conversation also includes a step-by-step explanation for deriving a common solution using partial fractions. The participants thank each other for their help and suggestions.
  • #1
maff is tuff
65
1

Homework Statement



∫ dx/(x² -a² )

Homework Equations



When (x² -a² ) appears in an integrand, you can use the trig sub: x=asecθ right?

The Attempt at a Solution



I know I could solve this using partial fractions but why doesn't trip sub work here? Or does it? I have attached my attempt below. Ignore the bottom half of the page. Thanks all for your help. :)

 

Attachments

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  • #2
Trig sub works. [tex]\int \csc\thetad\theta = -\ln|\csc\theta+\cot\theta|[/tex]
 
  • #3
How would I derive that? I would need to show how to get there to get any credit. Or what would be my first step in deriving that because I have no idea. And thanks for replying to one of my questions again; I recognize you from a couple nights ago.
 
  • #4
Change csc to (csc^2 + csccot)/(csc + cot). Then use u-substitution.
 
  • #5
A common way to do this integration is to expand 1/(x2 - a2) using partial fractions.

(x2 - a2) = (x - a)(x + a), therefore:

[tex]\frac{1}{x^2-a^2}=\frac{B}{x-a}\,+\,\frac{C}{x+a}[/tex]

Multiply both sides by (x - a)(x + a). Find B & C.

Your integral then becomes: [tex]\int\,\left(\frac{B}{x-a}\,+\,\frac{C}{x+a}\right)\,dx[/tex]
 
  • #6
Thank you all for your replies. I will try your suggestions.
 

Related to How to Use Trig Substitution for Integrals Involving (x²-a²)

What is a trigonometric substitution?

A trigonometric substitution is a method used to solve integrals involving expressions with a square root of a sum of squares. It involves substituting a trigonometric function for a variable in the integral that will make the integral easier to solve.

When should I use partial fractions?

Partial fractions are used when we have a rational function, which is a function that is a ratio of two polynomials. It helps us to break down a complex rational function into simpler fractions that can be easily integrated.

What are the steps for solving a trigonometric substitution?

The steps for solving a trigonometric substitution include: 1. Identify the expression inside the square root.2. Choose the appropriate substitution for the variable.3. Rewrite the integral in terms of the new variable.4. Use trigonometric identities to simplify the integral.5. Solve the resulting integral using traditional integration techniques.6. Substitute the original variable back in the solution.

Do I need to memorize all the trigonometric identities for trigonometric substitution?

No, you do not need to memorize all the trigonometric identities for trigonometric substitution. It is helpful to have a basic understanding of the common identities, but they can also be easily looked up when needed.

What are some common mistakes to avoid when using trigonometric substitution?

Some common mistakes to avoid when using trigonometric substitution include: 1. Choosing the wrong substitution for the variable.2. Forgetting to substitute back in the original variable in the solution.3. Forgetting to use the appropriate trigonometric identities to simplify the integral.4. Making calculation errors when solving the integral.5. Forgetting to check for extraneous solutions in the final answer.

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