# Homework Help: Integral, correct?

1. Nov 11, 2007

### rocomath

anyone have time to waste? lol, i've been trying to check it with the derivative but either i'm messing up or it's wrong, but i just feel like i did it right ... i'm just having a hard time computing

also, did i take the right approach? thanks.

Last edited: Nov 12, 2007
2. Nov 11, 2007

### Dick

It's not really clear what integral you are actually trying to solve. Is there really a t^2 in the exponent? Is the sinh multiplying that?

3. Nov 11, 2007

### rocomath

sorry, the parenthesis is meant for a substitution

$$\int e^{-t^2}dt$$

4. Nov 11, 2007

### Dick

You can't find an indefinite integral of e^(-t^2) in terms of elementary functions. The indefinite integral is called 'erf' (modulo constants).

Last edited: Nov 11, 2007
5. Nov 11, 2007

### rocomath

damn.

6. Nov 12, 2007

### dynamicsolo

I've got a separate question about this: is that upper limit really 'ln 1'? Wouldn't that make the two limits of integration identical? Is someone pulling your leg on this one? Is it a trick question?

7. Nov 12, 2007

### rocomath

it's from the MIT OCW website

http://ocw.mit.edu/NR/rdonlyres/6055BD0B-FEA6-4BBD-AB05-9E9D81615CAA/0/ocw01exam3.pdf [Broken]

lol it is both 0, i didn't even notice.

Last edited by a moderator: May 3, 2017
8. Nov 12, 2007

### dynamicsolo

Bwah-hah-hah! As a classical MIT nerd would declare, "You've been hacked!" (That does explain why that question's only worth 3 points. The lecturer was probably checking to see who was awake...)

Whoa! Good ol' 18.01 -- takes me back...

ProTip: Watch out if someone at MIT has something to do with an exam like this. They love to pull a fast one on the unwary at some point... (It's a place where parties unknown break into a "secure" area and adorn the Great Dome of the main building with something monumental almost every year.* They also measure bridge spans in smoots... What can you expect?)

*In 2003, to mark the centennial of heavier-than-air aviation, a rough reproduction of a Wright Flyer appeared atop the building overnight...

Last edited by a moderator: May 3, 2017
9. Nov 12, 2007

### Gib Z

Thats an MIT test? I don't believe you. Here I was thinking MIT was an elite University...what grade is test meant to be for?

10. Nov 12, 2007

### HallsofIvy

"18.01" is (college) freshman calculus. Every freshman at M.I.T. either "places" in a higher math course or takes 18.01 first semester freshman year (that's my memory from way back). I wouldn't want to be "elitist" but if you look carefully at that test you will see many problems that are easy if you understand the concepts rather than just apply formulas.

11. Nov 12, 2007

### rocomath

um yeah, you're definitely a nerd, lol. surprisingly that i did notice that it was "worth" 3 points. but since i didn't notice the lower/upper limits, it didn't click.

12. Nov 12, 2007

### rocomath

13. Nov 12, 2007

### dynamicsolo

Sorry, I wouldn't generally post something like that, but when I say that the "trick question" was from an MIT hour exam, it was hard to resist dropping into character.

I think that part of the point of these kinds of conceptual problems is to get students into the habit of looking for key ideas or peculiar details that may make a difficult-looking problem easy to solve -- certainly a valuable skill.

14. Nov 12, 2007

### rocomath

ah i'm just teasin. i have a lot of respect for ppl's help. anyways, i'm glad i made that dumb mistake, i'll def. be more aware from now on. ;-]