# Euler substitution (Good sources)

1. Jul 21, 2011

Both online and books would be suffice. Also videos, if there are any.

All help would be greatly appreciated =)

2. Jul 21, 2011

### MathematicalPhysicist

Shouldn't it be in any integral and differential calculus books?

3. Jul 21, 2011

Well it is not in my book =(

And ofcourse different books explain things in different ways. Some good and some horrendous. So I was wondering if anyone knew about a book having a clear, good explenation of it.

4. Jul 21, 2011

### nickalh

Are you meaning integrals and derivatives with e^x, e^u, ln x & ln u ?

A solid grasp of the substitution guidelines is a great starting point.

Are you looking principles like, when integrating,
Choose the most complex u, which has the derivative of u multiplied elsewhere in the integrand.
?

I don't or rarely see those principles written in textbooks. Usually, the professor or a tutor is the best source for guidelines. Most colleges have free tutoring clinics available with varying instructional quality. Several in my area will tutor students from other colleges.

5. Jul 22, 2011

I was talking about three substitutions, I thought they were called euler substitutions. But I had a hard time finding them left.

$$\sqrt{ax^2+bx+c}=t-\sqrt{a}x$$

$$\sqrt{ax^2+bx+c}=xt + \sqrt{c}$$

$$\sqrt{ax^2+bx+c}=x(t-c)$$'

Sorry about my english, nt sure what these are called.

6. Jul 22, 2011

### nickalh

Again give us a context.
Are these for evaluating integrals? For differential equations? Something else like cubic and higher degree equations?

7. Jul 22, 2011

Yeah, they are for evaluating integrals. Called euler substitutions, or atleast I think they are calle thath.

I sort of want to know when these are useful, when to preffer one over the other one. When to use this over for example trigonometric substitution. Also wondering if there are any integrals that "must" be solved by this method

8. Jul 22, 2011

### disregardthat

I think what you may be looking for is Abel substitution. It is basically t = sqrt(ax^2+bx+c). After that a simple linear transformation will give your examples.

9. Jul 22, 2011

### nickalh

I doubt you'll find an entire book on it. One might find a section using it or an entire chapter using it.
I google Able substitution which led me to Abel transforms. Not much success there.
Can you give me some example integrals you're trying to evaluate?
I can probably figure out application methods with more details.

Neither
http://www.wolframalpha.com/input/?i=integral+1%2F+root+%28ax^2%2Bbx%2Bc%29+dx
nor
http://www.wolframalpha.com/input/?i=integral+root+%28ax^2%2Bbx%2Bc%29+dx
are known to WolframAlpha, the most extensive, free computer integrator that exists. So I need at least a few examples to help you.

10. Jul 22, 2011

### nickalh

Correction to previous post:
http://www.wolframalpha.com/input/?i=integral+%28ax^2%2Bbx%2Bc%29+^.5dx
and
http://www.wolframalpha.com/input/?i=integral+1%2F+%28ax^2%2Bbx%2Bc%29+^.5dx

both evaluate. The show steps feature on the first link shows the substitution and uses one of yours. Notice how WolframAlpha, abbreviated W|A, first completes the square, then does the substitution?
So we've got a way to see how it works in practice. Perhaps ask WolframAlpha enough integrals and you figure out the how and why?

As to when to use these vs. when to use trig. substitution, trig. substitution on $\sqrt{trinomial}$ is probably even uglier than the method W|A shows. So wouldn't it make sense to use these for $\sqrt{trinomial}$, but trig. substitution for forms similar to $\sqrt{1\pm x^2}$?

I've reached the limit of how I can help. God bless you & bye.

11. Jul 22, 2011

### nickalh

One more thought occurred to me. The choice of which of the three t expressions to substitute will be influenced by what makes the square root real? Unless, you've learned or are learning calculus of complex numbers.