Help with integrating arctan function

In summary, the student is trying to solve a problem in Calc II that he was unable to solve. He was helped by another student who explained the solution more fully.
  • #1
ssmith147
3
0

Homework Statement


INTEGRATE dx / (2 * root(x)) * (1 + x)



Homework Equations


That's pretty much it!


The Attempt at a Solution


I received this question on a Calc II exam, so I'm only looking for the solution for my own understanding (I'm sure I already got it wrong). My instinct is that this is an arctan integral in the form of 1 / 1 + u^2. Unfortunately, after playing around with it for about 30 minutes I was unable to find a way to get the denominator into the form 1 + u^2.

Am I missing something or did I misjudge the solution?

Anyw help would be greatly appreciated!
 
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  • #2
What if you use [tex]\sqrt{x}[/tex] as u?

What does du/dx become, and then the rest of the equation?
 
  • #3
u = root(x) so du = 1/(2*root(x)).

INT [1 / ( 2 * root(x))] * [1 / (1 + x)] dx
= INT [1 / (1 + x)] du = ln|1 + x|

That doesn't seem to work, though. If I differentiate this function I only get the original function with respect to u, not with respect to x since the value of u is eliminated by the value of du during the substitution. I've tried playing with this but I'm not seeing a way to successfully balance the value of du and retain u in the function.

Am I missing the obvious here?
 
  • #4
You need to express ∫[1 / (1 + x)] du in terms of u with no x at all since you have to integrate with respect to u. So using the substitution u = √x → x = u2, then that gives you ∫du/(1 + u2) which you can now integrate correctly. :smile:
 
  • #5
Wow, I can't believe I didn't see that. During the exam I tried using root(x) for u but I never thought about root(x)^2 = x. Tunnel vision must have set in- I should have seen that!

So the answer would be arctan(root(x))- I suppose I can console myself knowing I saw arctan(x) correctly. Devil's in the details, unfortunately.

I really appreciate the feedback on this. At least I know what I did wrong now!
 
  • #6
That's it. Knowing (or figuring out) what to make u in these cases can be a real pain in the neck. And in some cases - such as this - it may not be particularly obvious whether simple substitution or integration by parts is necessary.

I was out last night, so thanks to Bohrok for explaining it more thoroughly.
 

FAQ: Help with integrating arctan function

What is the arctan function and how is it different from the tan function?

The arctan function, also known as the inverse tangent function, is the inverse of the tangent function. It takes the output of the tangent function and finds the angle that produced that output. This is different from the tangent function which takes an angle and outputs the ratio of the opposite side to the adjacent side in a right triangle.

How do I integrate the arctan function?

To integrate the arctan function, you can use the substitution method. Let u = tan(x) and then use the formula ∫arctan(u) du = u*arctan(u) - 1/2*ln|1+u^2| + C. Don't forget to substitute back in u = tan(x) at the end.

3. Can the arctan function be integrated using other methods?

Yes, the arctan function can also be integrated using integration by parts or partial fraction decomposition. However, these methods can be more complex and may require additional algebraic manipulation.

4. Are there any special cases when integrating the arctan function?

Yes, there are a few special cases when integrating the arctan function. One case is when the argument of the arctan function is a constant. In this case, the integral simplifies to a linear function. Another special case is when the argument of the arctan function is a trigonometric function, such as cos(x) or sin(x). In these cases, you can use trigonometric identities to simplify the integral.

5. Can the arctan function be used in real-world applications?

Yes, the arctan function is commonly used in physics and engineering to calculate angles and solve problems involving right triangles. It is also used in signal processing and control systems to calculate phase angles.

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