Stuck with an integral

  • #1
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Main Question or Discussion Point

Hello everyone

I need some help in performing the following integration (not HW):

[tex]\int_{0}^{\pi/4}\left(\frac{x}{x\sin x + \cos x}\right)^{2}dx[/tex]

I tried integration by parts, but it leads nowhere. Any suggestions would be appreciated.

Thanks
Vivek
PS--Mathematica gives the answer as [itex](4-\pi)/(4+\pi)[/itex] but is unable to perform the integration with the [itex]x^2[/itex] term in the numerator replaced by unity.
 

Answers and Replies

  • #2
To solve your integral, you can start by differentiating

[tex]-\frac{x \sec{x}}{(x \sin{x} + \cos{x})}[/tex]

this will give you

[tex]-(\frac{x \sec{x} }{x \sin{x} + \cos{x}})^{\prime} = \frac{x^2}{(x \sin{x} + \cos{x})^2} - \frac{(\sec{x} + x \sec{x} \tan{x})}{(x \sin{x} + \cos{x})}[/tex]

Now, you'll recognize the first term on the right hand side as your integrand. To evaluate the second term on the right hand side, you can first take the [tex]\sec{x}[/tex] out of the top half to get

[tex]\frac{(\sec{x} + x \sec{x} \tan{x})}{(x \sin{x} + \cos{x})} = \sec{x} \frac{1 + x \tan{x}}{(x \sin{x} + \cos{x})}[/tex]

Now, divide the bottom half by [tex]\cos{x}[/tex] to get

[tex]\frac{\sec{x}}{\cos{x}} \frac{(1 + x \tan{x})}{(x \tan{x} + 1)} = \sec^2{x}[/tex]

Now, the integral of [tex]\sec^2{x}[/tex] is

[tex]\int{\sec^2{x} dx} = \tan{x} + C [/tex]

therefore, your integral, is

[tex]\int{(\frac{x}{x \sin x + \cos x})^{2} dx} = \tan{x} - \frac{x \sec{x}}{(x \sin{x} + \cos{x})} + C[/tex]

where C is a constant of integration.
 
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  • #3
Gib Z
Homework Helper
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I'm not sure about you Matthew but most people don't have a preset function they know the should differentiate and compare their integral to. Otherwise one might just say to differentiate [tex]\frac{\sin x - x\cos x}{\cos x - x\sin x}[/tex] and see what you get.
 
  • #4
Fair enough, but it wasn't a guess -- I got there by trying to find out what function, f(x), when divided by [tex](x \sin{x} + \cos{x})[/tex] yields the integrand in part of its derivative. (The answer of course is [tex]f(x) = -x \sec{x}[/tex]). Then it's a question of seeing if you can integrate the other part(s) of the derivative -- if you can, you have a solution. In this case, it was possible.
 
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  • #5
Gib Z
Homework Helper
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In that case, good work mate :)
 
  • #6
No, you're right -- I should have made this clear at the start. Sorry folks.
 
  • #7
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Thanks Matthew and GibZ

Sorry for the late acknowledgement...I figured out how to do it, by a method similar to that suggested.

GibZ, if I take your function with the minus replaced by plus in the denominator, then the derivative equals the integrand. So thats an equivalent way of doing it.

Thanks again
 
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