# Homework Help: Challenged by my teacher: integration by substitution

1. Feb 26, 2008

### Juan Pablo

I just learned how to integrate through substitution and I was challenged by my teacher with an apparently easy problem but I'm really struggling with it.
He said he will give me an F if I don't solve it for tomorrow, I guess this is what I get by being the one who always understand in class

Exposed below

1. The problem statement, all variables and given/known data

$$\int (x^2+1)^2 dx$$

2. Relevant equations

$$x^2+1=u$$

$$\frac{du}{2x}=dx$$

$$x=\sqrt{u-1}$$

3. The attempt at a solution

$$\int u^2(\frac{du}{2x}) =$$

$$\int u^2(\frac{du}{2\sqrt{u-1}}) =$$

$$\frac{1}{2}\int\frac{du}{u^{-2}\sqrt{u-1}} =$$

Now I finally integrate

$$\frac{1}{2} ln(u^{-2}\sqrt{u-1})$$

$$\frac{1}{2} ln(\frac{\sqrt{u-1}}{u^2})$$

$$\frac{1}{2} ln(\frac{x}{(x^2+1)^2})$$

But if I integrate it solving the binomial first I get a totally different answer and If I derive my result I don't get $$(x^2+1)^2$$

Anybody?

Last edited: Feb 26, 2008
2. Feb 26, 2008

### PingPong

You can check your answer by just foiling out $(x^2+1)^2$. So you really shouldn't be getting natural logs...

Perhaps he's just trying to get you to use u substitution when you don't have to?

3. Feb 26, 2008

### Juan Pablo

Yes, that was my first thought but he said that I must use substitution, and this is the only way I think its possible.

4. Feb 26, 2008

### olgranpappy

But, that is wrong. That is not the value of the integral.
you get a different answer because you get the right answer, which is different from the wrong answer.

5. Feb 26, 2008

### Juan Pablo

I'm just learning this, is there anybody out there willing to show me how to integrate it correctly through substitution?

6. Feb 26, 2008

### Hurkyl

Staff Emeritus
That's clearly not right.

7. Feb 26, 2008

### Juan Pablo

Isn't this one of the formulas?

$$\int \frac{dx}{x} = ln(x)$$

I'm really trying, and I would appreciate answers with more than one line. I know it's wrong but if somebody would be so kind and try to explain it to me?

I just learned integration three days ago and I learned substitution this morning

Last edited: Feb 26, 2008
8. Feb 26, 2008

### PingPong

Hi Pablo,

Sorry if it seems that we're being a bit short with our answers. The homework forum isn't a place where others do your homework for you, just offer advice.

In any case, You're right that $\int \frac{dx}{x} = \ln(x)$ - but this isn't what you have, is it? Instead, you have $\int \frac{dx}{x^{-2}\sqrt{1-x}}$ - that is, $\int \frac{dx}{f(x)} \ne \ln(f(x))[/tex] in general. As for how to integrate that thing, I'm afraid I'm stumped without further substitution. Integrals.com and the back of Stewart's calculus book gives [itex]2/30 \sqrt{u-1}(3u^2+4u+8)$ the answer, which, when substituting back in your original substitution gives the correct answer.

EDIT: Stewart's Calculus has the following:

$$\int \frac{u^2du}{\sqrt{a+bu}}=\frac{2}{15b^3}(8a^2+3b^2u^2-4abu)\sqrt{a+bu}+C$$.

For your integral, $a=b=1$

Last edited: Feb 26, 2008
9. Feb 26, 2008

### cristo

Staff Emeritus
Why would you think of trying to solve that integral by substitution? Any substitution (at least that I can think of) would just make the integrand more complicated. If I were you, I'd just expand and solve it in two lines (of course you can make the substitution u=x if your teacher demands a substitution ;) ) and move on to trying to solve more complicated problems that will help you hone your techniques for the future.

10. Feb 26, 2008

### Juan Pablo

Yes I know you are not doing my homework, and it's not really my homework, it's a challenge put by my teacher just to me. I'm also a permanent user in other forums about technical topics and I always try to be as detailed and kind as possible when I answer questions. Even If they are homework I give hints and tips to the poster.

Mathematica gave me the same answer, thank you however

Cristo: Yes, it's a lot easier without substitution and I can solve it that way right now, but I'm asked to use substitution. I think I'll stick to u=x like you said. Thanks

11. Feb 26, 2008

### Juan Pablo

Thanks for your edit and formula, I think I will use it although I haven't seen that formula on class.

12. Feb 26, 2008

### Hurkyl

Staff Emeritus
You can't apply that rule -- $du / (u^{-2}\sqrt{u-1})$ simply doesn't match the pattern $dx / x$.

Last edited: Feb 26, 2008
13. Feb 26, 2008

### Juan Pablo

My teacher seems to be a moron, he used the formula a couple of times like I did. I think this led to my confusion. I believe the correct way is with PingPong's formula

14. Feb 26, 2008

### Juan Pablo

Thanks Hurkyl PingPong, your help is really appreciated

15. Feb 27, 2008

### cristo

Staff Emeritus
But the point of substitutions is to make an integral simpler to compute, not make it harder! If your teacher did intend that to be the solution, then I have no idea what he is thinking-- what a waste of time!