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Cruising the old questions I came across juantheron asking for $\int \frac {\sec x\tan x}{3x+5}\,dx$ He tried using $(3x+5)^{-1}$ for $U$ and $\sec x \tan x$ for $dv$while integrating by parts. below is his work.

How can I calculate $$ \int {\sec\left(x\right)\tan\left(x\right) \over 3x + 5}\,{\rm d}x $$

My Try:: $\displaystyle \int \frac{1}{3x+5}\left(\sec x\tan x \right)\,\mathrm dx$

Now Using Integration by Parts::

We get

$$= \frac{1}{3x+5}\sec x +\int \frac{3}{(3x+5)^2}\sec x\,\mathrm dx$$

Here he hit his road block.

I tried the opposite tactic

Taking the other approach by parts.

let $$U= \sec x \tan x$$ then$$ du= \tan^2 x \sec x +\sec^3 x$$ and $$dv=(3x+5)^{-1}$$ then $$v=\frac 1 3 \ln(3x+5)$$ Thus $$\int \frac {\sec x \tan x}{3x+5}\,dx= \frac {\ln(3x+5)\sec x \tan x}{3} - \int \frac {\ln(3x+5) [\tan^2 x \sec x +\sec^3 x]}{3} \,dx$$

As you can see I got no further than he did.

So how many times do you have to complete integration by parts to get the integral of the original $\frac {\sec x \tan x}{3x+5} \, dx$ or is there a better way?

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4  
It's entirely possible that this function has no elementary antiderivative. In other words, there's no reason to expect that you can find the answer. –  Greg Martin Dec 25 '13 at 22:18
    
duplicate of math.stackexchange.com/questions/395271/… –  Greg Martin Dec 25 '13 at 22:19
    
@GregMartin why is it a duplicate if OP clearly mentions it in the question? –  Ian Mateus Dec 25 '13 at 22:22
    
@GregMartin Not exactly a duplicate. I did have to include the original post in my question or it would have been plagiaristic. Also to explain why I took my approach. Honestly it is a separate "challenge" to solve and old question I found interesting. –  Chris Dec 25 '13 at 22:26
    
@GregMartin So for all intents and purposes this could be like trying to find the last digit of ${\pi}$? Can anyone find a way? Or prove their is no elementary integral? –  Chris Dec 25 '13 at 22:30

2 Answers 2

up vote 3 down vote accepted

Integrating elementary functions in elementary terms is completely algorithmic. The algorithm is implemented in all major computer algebra systems, so the fact that Mathematica fails to integrate this in closed form can be viewed as (in effect) a proof that such a closed form does not exist in elementary terms.

To answer the comments:

  1. You may or may not trust Mathematica (I don't always, but do for this). The fact that "its algorithm is not open for inspection" is not relevant -- it would take you much longer to figure out what the code does than to run the algorithm by hand (well, you need some linear algebra).

  2. If you do want to try it in the privacy of your own home, you need to go no further than the late, lamented Manuel Bronstein's excellent book: http://www.amazon.com/Symbolic-Integration-Transcendental-Computation-Mathematics/dp/3540214933

  3. I am quite sure that this particular integral is easy to show non-integrable in elementary terms by hand (if you understand the Risch algorithm).

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Take care. Indeed, from that page: "The Risch algorithm applied to general elementary functions is not an algorithm but a semi-algorithm because it needs to check [...] if certain expressions are equivalent to zero [...]. For expressions that involve only functions commonly taken to be elementary it is not known whether an algorithm performing such a check exists or not (current computer algebra systems use heuristics); moreover, if one adds the absolute value function to the list of elementary functions, it is known that no such algorithm exists; see Richardson's theorem." –  Sharkos Dec 26 '13 at 2:48
2  
While Mathematica's failure to integrate is probably good enough for practical purposes, the fact that its algorithm is not open for inspection means that it cannot be considered a reliable proof. –  Greg Martin Dec 26 '13 at 2:51

Finally figured this one out.

${let: 3x+5= \sec \theta, x=\frac {\sec\theta+5}{3}, dx=\frac{1}{3}\sec\theta\tan\theta d\theta}$

then we have $${\frac {1}{3}\int\frac{\sec\frac{\sec\theta+5}{3}\tan\frac{\sec\theta+5}{3}}{\sec\theta}\sec\theta\tan\theta d\theta=}$$ $${\frac{1}{3}\int\cos\theta {\sec\frac{\sec\theta+5}{3}\tan\frac{\sec\theta+5}{3}}\sec\theta\tan\theta d\theta}$$ Then

${let: u=\frac{\sec\theta+5}{3}=, du=\frac{1}{3}\sec\theta\tan\theta d\theta}$ $${\frac {1}{9}\cos\theta\int\sec u \tan u du=}$$ $${\frac{1}{9}\cos\theta\sec u+c=}$$ $${\frac{1}{9}\frac{\sec x}{3x+5}+c}$$

${Edit}$

Nope back to the drawing board.

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1  
$\cos \theta$ cannot come out of the integral. –  i707107 Jan 20 at 1:20
    
You can express $\cos \theta$ as a function of $u$, and then you get back with $\int \frac{\sec (u) \tan (u) }{3u-5}du$. –  Olivier Jan 20 at 2:13
    
I can eliminate ${\cos\theta}$ before the second substitution that leaves me with $${\frac{1}{3}\int {\sec\frac{\sec\theta+5}{3}\tan\frac{\sec\theta+5}{3}}\tan\theta d\theta}$$ That still leaves me with $${\frac{1}{3}\int \sec u\tan u\cos\theta du}$$ so that gets me nowhere. Would a laplace transform help with this? –  Chris Jan 20 at 3:18

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