Can Trigonometric Substitution Solve This Integral?

In summary, the conversation discusses how to solve an integral involving a complex expression. Suggestions are made to use trigonometric substitution and partial fraction decomposition, but it is ultimately determined that the integral has no elementary anti-derivative. Maple is used to obtain a closed-form solution involving elliptic functions.
  • #1
askor
169
9

Homework Statement



Please tell me how to solve the below integral

##\int \frac{(1+x^2)}{(1-x^2)\sqrt{1+x^4}}dx##

Homework Equations

The Attempt at a Solution



This is beyond my imagination so, believe me or not, I can't attempt anything!

Give me hint please.

I guess the denominator can be solve by using trigonometric substitution but the x is in ##x^4## form, not ##x^2##. So, it can not be solved using the trigonometric substitution.
 
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  • #2
What makes you think this integral has a simple anti-derivative? :sorry:
 
  • #3
Mathematica gives me the following:
$$\sqrt[4]{-1} \left(F\left(\left.i \sinh ^{-1}\left(\sqrt[4]{-1} x\right)\right|-1\right)-2 \Pi \left(i;\left.\sin ^{-1}\left((-1)^{3/4} x\right)\right|-1\right)\right)$$ where [itex]F[/itex] and [itex]\Pi[/itex] are elliptic functions. If this is correct than (to the best of my knowledge) this integral has no elementary anti-derivative (but, as any integral, can be approximated numerically to arbitrary precision if needed).
 
  • #4
askor said:

Homework Statement



Please tell me how to solve the below integral

##\int \frac{(1+x^2)}{(1-x^2)\sqrt{1+x^4}}dx##

Homework Equations

The Attempt at a Solution



This is beyond my imagination so, believe me or not, I can't attempt anything!

Give me hint please.

I guess the denominator can be solve by using trigonometric substitution but the x is in ##x^4## form, not ##x^2##. So, it can not be solved using the trigonometric substitution.

If I use Maple on the definite-integral version, I get a simple, closed-form answer involving only elementary functions, but if I ask Maple to perform the indefinite integral I get a nasty formula involving Elliptic functions, similar to what ELB27 obtained.

Here is the definite-integration version. If ##f(x)## is your integrand and ##F(x) = \int_0^x f(t) \, dt##, then
[tex] F(x) = \frac{1}{\sqrt{2}} \text{arctanh} \left(\frac{\sqrt{1+x^4}\sqrt{2}}{2x} \right) + \frac{1}{4} i \pi \sqrt{2} [/tex]
Despite its complex appearance, the formula for ##F(x)## returns real numerical values for real values of ##x \geq 0##. Also: ##dF(x)/dx = f(x)##, as it should!
 
  • #5
Ray Vickson said:
If I use Maple on the definite-integral version, I get a simple, closed-form answer involving only elementary functions, but if I ask Maple to perform the indefinite integral I get a nasty formula involving Elliptic functions, similar to what ELB27 obtained.

Here is the definite-integration version. If ##f(x)## is your integrand and ##F(x) = \int_0^x f(t) \, dt##, then
[tex] F(x) = \frac{1}{\sqrt{2}} \text{arctanh} \left(\frac{\sqrt{1+x^4}\sqrt{2}}{2x} \right) + \frac{1}{4} i \pi \sqrt{2} [/tex]
Despite its complex appearance, the formula for ##F(x)## returns real numerical values for real values of ##x \geq 0##. Also: ##dF(x)/dx = f(x)##, as it should!
Interesting... Then it must be a matter of simplifying the elliptic functions somehow to get to your answer.
 
  • #6
ELB27 said:
Interesting... Then it must be a matter of simplifying the elliptic functions somehow to get to your answer.

I guess that must be what is happening, but I really don't know. In particular, the actual technique remains a mystery to me.
 
  • #7
Ray Vickson said:
I guess that must be what is happening, but I really don't know. In particular, the actual technique remains a mystery to me.
I haven't attempted to solve it, but how I would start is maybe by looking at the denominator first. 1-x^2 is a difference of squares, so write it out as (1+x)(1-x) and cancelling the top and one of the factors on the bottom. Then, the integral becomes ∫1/[(1-x)(√1+x^4)]dx
Then, I would probably use trig sub and set x = to either sin(θ)^2 or tan(θ)^2, or maybe try using trig and then u sub, I haven't sat down to really look at it and try, I'm outta time, I'm sorry, but that should help a little at least.
 
  • #8
PhysixChick said:
I haven't attempted to solve it, but how I would start is maybe by looking at the denominator first. 1-x^2 is a difference of squares, so write it out as (1+x)(1-x) and cancelling the top and one of the factors on the bottom. Then, the integral becomes ∫1/[(1-x)(√1+x^4)]dx
Then, I would probably use trig sub and set x = to either sin(θ)^2 or tan(θ)^2, or maybe try using trig and then u sub, I haven't sat down to really look at it and try, I'm outta time, I'm sorry, but that should help a little at least.
Um ... ##1+x^2## is not canceled by ##1+x## .
 
  • #9
SammyS said:
Um ... ##1+x^2## is not canceled by ##1+x## .
Oh shoot, I misread that... my bad.
 
  • #10
OK, i figured it out. It's partial fraction decomposition bro. First, split up (1+x^2) as
∫ 1 / [ (1+x) (1-x) (1+x^4)^1/2] dx and do partial fraction decomposition for that.
Then, do ∫ x^2 / [(1+x) (1-x) (1+x^4)^1/2] dx and do partial fractions again.
Then, add the results.
Let me know if that works, yeah?
 
  • #11
PhysixChick said:
OK, i figured it out. It's partial fraction decomposition bro. First, split up (1+x^2) as
∫ 1 / [ (1+x) (1-x) (1+x^4)^1/2] dx and do partial fraction decomposition for that.
Then, do ∫ x^2 / [(1+x) (1-x) (1+x^4)^1/2] dx and do partial fractions again.
Then, add the results.
Let me know if that works, yeah?

You cannot "split up" ##1+x^2## into partial fractions, because it is in the numerator (although, of course, it is already "split up" because it consists of two terms added together, and you can write two separate integrals). You can split up ##1-x^2## into partial fractions, because it is in the denominator. However, whether or not that helps is the real issue. It was your suggestion, so why don't you try it and report back on whether it works?
 
  • #12
Ray Vickson said:
You cannot "split up" ##1+x^2## into partial fractions, because it is in the numerator (although, of course, it is already "split up" because it consists of two terms added together, and you can write two separate integrals). You can split up ##1-x^2## into partial fractions, because it is in the denominator. However, whether or not that helps is the real issue. It was your suggestion, so why don't you try it and report back on whether it works?
not

because it's not my homework. i thought about it, i don't know why i said split it up, just straight up use partial fractions. worked for me.
 
  • #13
I worked backwards from the answer Ray posted above, and I found the substitution
$$ u = \frac{\sqrt{2} x}{\sqrt{1+x^4}} $$ will result in
$$\frac{1}{\sqrt{2}} \int \frac{1}{1-u^2}\,du.$$ The integral ends up equal to
$$\frac{1}{2\sqrt 2} \log\frac{1+\frac{\sqrt 2 x}{\sqrt{1+x^4}}}{1-\frac{\sqrt 2 x}{\sqrt{1+x^4}}}.$$
 
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  • #14
PhysixChick said:
not

because it's not my homework. i thought about it, i don't know why i said split it up, just straight up use partial fractions. worked for me.

The statement "worked for me" means that you actually did it and so obtained a final formula for the integral. Right?
 
  • #15
I'm just tryna give insight. and yeah, i got an answer.

(√2 / 2) (ln|1+x|) - (1/2) (ln|1-x|) - (1/2 - √2/2) √{ [ ( 1 + ( √x ) ] ^2 + 1}

if this ain't right, there's nothing simple that can be done by hand. not all integrals have an antiderivative (for example, e^x^2)
 
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  • #16
PhysixChick said:
I'm just tryna give insight. and yeah, i got an answer.

(√2 / 2) (ln|1+x|) - (1/2) (ln|1-x|) - (1/2 - √2/2) √{ [ ( 1 + ( √x ) ] ^2 + 1}

if this ain't right, there's nothing simple that can be done by hand. not all integrals have an antiderivative (for example, e^x^2)

The answer is definitely not right: the derivative of the above does not match the original integrand.

Of course some functions do not have elementary antiderivatives; that was the main issue in this whole thread---the fact that some powerful integrators, such as Mathematica produce result involving Elliptic functions, while some runs of Maple produce Elliptic integral answers but others give an elementary antiderivative. However, none of the obvious elementary calculus tools suffice in this example. I am still trying to get Maple to spit out the algorithms it used to get an elementary answer; I think it at least started to use the initial portions of Risch's method, but I am not sure.
 
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  • #17
Ray Vickson said:
The answer is definitely not right: the derivative of the above does not match the original integrand.

Of course some functions do not have elementary antiderivatives; that was the main issue in this whole thread---the fact that some powerful integrators, such as Mathematica produce result involving Elliptic functions, while some runs of Maple produce Elliptic integral answers but others give an elementary antiderivative. However, none of the obvious elementary calculus tools suffice in this example. I am still trying to get Maple to spit out the algorithms it used to get an elementary answer; I think it at least started to use the initial portions of Risch's method, but I am not sure.

Oh I didn't forget that, your moderator friend deleted it.
 
  • #18
PhysixChick said:
Oh I didn't forget that, your moderator friend deleted it.
I don't see that any of your work was deleted -- just your insult.
 
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  • #19
Ray Vickson said:
Here is the definite-integration version. If ##f(x)## is your integrand and ##F(x) = \int_0^x f(t) \, dt##, then
[tex] F(x) = \frac{1}{\sqrt{2}} \text{arctanh} \left(\frac{\sqrt{1+x^4}\sqrt{2}}{2x} \right) + \frac{1}{4} i \pi \sqrt{2} [/tex]
Despite its complex appearance, the formula for ##F(x)## returns real numerical values for real values of ##x \geq 0##. Also: ##dF(x)/dx = f(x)##, as it should!
The hyperbolic tangent (as a real valued function) has a range of (-1, 1) .

The argument of the above inverse hyperbolic tangent is strictly in the interval [1, ∞) . That helps explain the complex appearance even though the expression evaluates to a real result for any positive x.

If one uses the inverse hyperbolic cotangent rather than than tangent this works out a bit nicer. Notice that the derivatives of the two functions have the same form, but the domains are different.

Thus, the solution could be written:
##\displaystyle\ F(x) = \frac{1}{\sqrt{2}} \text{arccoth} \left(\frac{\sqrt{1+x^4}\sqrt{2}}{2x} \right) \ ##​
 
  • #20
SammyS said:
The hyperbolic tangent (as a real valued function) has a range of (-1, 1) .

The argument of the above inverse hyperbolic tangent is strictly in the interval [1, ∞) . That helps explain the complex appearance even though the expression evaluates to a real result for any positive x.

If one uses the inverse hyperbolic cotangent rather than than tangent this works out a bit nicer. Notice that the derivatives of the two functions have the same form, but the domains are different.

Thus, the solution could be written:
##\displaystyle\ F(x) = \frac{1}{\sqrt{2}} \text{arccoth} \left(\frac{\sqrt{1+x^4}\sqrt{2}}{2x} \right) \ ##​

I also got this, but did not mention it before.

It is obtained automatically in Maple when we use the 'evalc' command on the previous result, under the assumption that z > 0 (which forces Maple to regard z as real rather than a general complex number). 'evalc' does a "complex evaluation", which means it finds the real and imaginary parts of a complex expression. This converts the preceding complex expression to another complex expression having zero imaginary part. The result is equivalent to yours, and the reasons are essentially the same.
 
  • #21
You can rewrite it as (x^-2 + 1)/{(x^-1 - x)sqrt[(x^-1 - x)^2+2]}
Then, substitute (sqrt2)tan u=x^-1 - 1
 
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  • #22
Momoko has the answer, just use his substitution then you will get it.
 
  • #23
There was a typo and I am unable to edit on the phone. The substitution should be (sqrt2)tan u=x^-1 - x
 
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  • #24
Yes, equivalently to momoko and in case you wonder however that substitution could be arrived at, then a key is a symmetry in the expression between x and 1/x

The integral is of (1 + 1/x2)dx/[(1/x -x)√(x2 + 1/x2)] =

= - d(x - 1/x)/[(1/x -x)√(x2 + 1/x2)] = - dz/[z√(z2 + 2)]. Where z = x - 1/x

Inviting another substitution y2 = z2 + 2
And we end up with essentially

∫dy/(y2 - 2) I think, which can then be expressed as partial fractions and integrated as a logarithmic or inverse hyperbolic function, I did a smilar one yesterday https://www.physicsforums.com/posts/5120268/ . Not sounding quite same as Momoko's but maybe there is a mistake of sign or something somewhere, at any rate you have enough to be able to work it out more carefully. (After you get and verify it we would be glad to see it here!).
 
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  • #25
Or just use a trigoniometric subtitution!
 

Related to Can Trigonometric Substitution Solve This Integral?

1. What is the process for solving a hard integral?

To solve a hard integral, you first need to determine the appropriate integration technique. This could include substitution, integration by parts, or trigonometric substitution. Then, you will need to rearrange the integral into a more manageable form and apply the chosen technique. Finally, you will need to evaluate the integral using the fundamental theorem of calculus or other appropriate methods.

2. How do I know which integration technique to use?

Choosing the right integration technique can be challenging, but there are some key factors to consider. Look for patterns or familiar forms in the integral, such as trigonometric functions, logarithmic functions, or polynomials. You can also try using integration by parts if there is a product of functions in the integral. Practice and experience will also help you become more familiar with which technique to use.

3. What are some common mistakes to avoid when solving a hard integral?

One common mistake when solving hard integrals is forgetting to apply the chain rule or product rule when differentiating. This can lead to incorrect integrals or an incorrect final answer. Another mistake is forgetting to include the constant of integration when evaluating the integral. It's important to double-check your work and be mindful of these common errors.

4. How can I simplify a complex integral?

If you encounter a complex integral, try using algebraic manipulation to simplify the expression before applying any integration techniques. You can also try breaking the integral into smaller parts and solving each part separately. Additionally, utilizing trigonometric identities or other mathematical identities can help simplify the integral.

5. Are there any tips for becoming better at solving hard integrals?

Practice is key when it comes to improving your skills in solving hard integrals. Make sure to review the fundamental techniques for integration and familiarize yourself with common functions and their integrals. Additionally, try working through a variety of problems to gain a better understanding of how to approach different types of integrals. You can also seek help from a tutor or study group to improve your skills and understanding.

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