# Homework Help: Limits of integration

1. Feb 25, 2008

### ehrenfest

[SOLVED] limits of integration

1. The problem statement, all variables and given/known data
I want to substitute x=t^2 in $\int_{-\infty}^{\infty}{\exp(-t^4)} dt$. What are the new limits of integration? They are both infinity aren't they? But the integral is clearly not zero? Is the problem that the substitution rule only holds for finite limits of integration?

2. Relevant equations

3. The attempt at a solution

2. Feb 25, 2008

### Mathdope

The substitution introduces dx = 2t dt within the integral. Perhaps a polar coordinate form would be helpful.

3. Feb 25, 2008

### ehrenfest

Of course, but $$2t = 2 \sqrt{x}$$. Why is this not valid:

$$\int_{-\infty}^{\infty}\exp(-t^4)dt = \int_{\infty}^{\infty}\exp(-x^2)dx/(2\sqrt{x})$$

?

4. Feb 25, 2008

### NateTG

Well, you need to use $-\sqrt{-x}$ where $x<0$.

5. Feb 25, 2008

### Mathdope

Maybe you should break the integral up into the part over t < 0 and t > 0. What does that turn into? I don't know the answer, just a suggestion.

6. Feb 25, 2008

### ehrenfest

I don't care how to evaluate the specific integral. My question is about the substitution rule. No one has yet explained to me why this

$$\int_{-\infty}^{\infty}\exp(-t^4)dt = \int_{\infty}^{\infty}\exp(-x^2)dx/(2\sqrt{x})$$

is not a valid use of the substitution rule?

7. Feb 25, 2008

### Mystic998

I think it's just because t^2 = x doesn't necessarily imply that t = sqrt(x).