How would you integrate this function?

In summary, the conversation discusses finding coefficients for a Fourier expansion and solving the integral (1/π) ∫sin(x/2)sin(nx)dx, where the limits are from -π to π and the original function is f(x) = sin(x/2). The approach is to use integration by parts, and the use of the Kronecker delta simplifies the integral. The conversation also mentions using complex numbers to solve the integral more easily.
  • #1
Vitani11
275
3

Homework Statement


I need to find the coefficients for a Fourier expansion. Here is the integral I need to solve: (1/π) ∫sin(x/2)sin(nx)dx where the limits are from -π to π. The original function for the expansion is f(x) = sin(x/2)

Homework Equations


None

The Attempt at a Solution


Should I do this in terms of complex numbers? I think that should be my approach but I am not sure. If this is the right approach can you help me get started with setting up the integral?
 
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  • #2
Vitani11 said:

Homework Statement


I need to find the coefficients for a Fourier expansion. Here is the integral I need to solve: (1/π) ∫sin(x/2)sin(nx)dx where the limits are from -π to π. The original function for the expansion is f(x) = sin(x/2)

Homework Equations


None

The Attempt at a Solution


Should I do this in terms of complex numbers?
I don't think so. The usual approach is integration by parts, twice. Here's a link to a similar example, https://www.math.ucdavis.edu/~kouba/CalcTwoDIRECTORY/trigintsoldirectory/TrigIntSol3.html#SOLUTION 24, problem #24
Vitani11 said:
I think that should be my approach but I am not sure. If this is the right approach can you help me get started with setting up the integral?
 
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  • #3
use the identity

$$\frac{1}{2\pi} \int_{-\pi} ^{\pi} \sin(nx)\sin(mx) dx = \delta_{nm}$$

and the integral pops right out for you . (i might have the normalization wrong... off the top of my head...)
 
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  • #4
Woah, so you're saying that using the kronecker delta I can just say it is δn (1/2)? Can you walk me through those steps? Does it have something to do with the dirac delta function/identities? If the kronecker truly makes it simpler then I'd rather use that lol.
 
  • #5
it is a definition...if you write the sine functions in terms of complex exponential's and do the integrals, it comes out very quickly...
 
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  • #6
$$\frac{1}{\pi}\int_{-\pi}^{\pi}\sin(nx)\sin(mx)dx = \delta_{nm}$$
 
  • #7
Yes I've solved this now- thank you
 

Related to How would you integrate this function?

1. What is the definition of integration?

Integration is a mathematical process that involves finding the area under a curve in a given interval. It is the inverse operation of differentiation and is commonly used in calculus to solve various problems in physics, engineering, and other fields.

2. How do you determine the limits of integration?

The limits of integration are determined by the given interval in which the function is to be integrated. These limits represent the starting and ending points of the interval and are denoted by lower and upper bounds respectively.

3. What are the different methods of integration?

There are several methods of integration, including substitution, integration by parts, partial fractions, and trigonometric substitution. Each method is used for different types of functions and can be chosen based on the complexity of the given function.

4. Can integration be used to solve real-world problems?

Yes, integration is a powerful tool that can be used to solve real-world problems in various fields such as physics, engineering, economics, and statistics. It is commonly used to find the area under a curve, calculate volumes of objects, and determine the rate of change of a variable.

5. What are the common mistakes to avoid when integrating a function?

Some common mistakes to avoid when integrating a function include forgetting to add the constant of integration, misapplying the chain rule, and making errors in algebraic manipulations. It is important to carefully check each step of the integration process to avoid these mistakes.

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