# Partial Derivatives

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## 15 Replies - 1182 Views - Last Post: 25 January 2011 - 08:43 AM

### #1 NeoTifa

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# Partial Derivatives

Posted 19 January 2011 - 05:50 PM

Okay, so I'm in Differential Equations, and she just starts talking about partial derivatives and shit, and I don't know what the fuck they are or how to solve them, and the test is tomorrow. She never explained them, kinda like she expected us to know about them. :x Help.
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## Replies To: Partial Derivatives

### #2 macosxnerd101

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## Re: Partial Derivatives

Posted 19 January 2011 - 06:04 PM

Basically, you differentiate for a single variable and treat everything else as constant. So if you have x^2 + 3xy + y, and you take a partial derivative wrt x, you get 2x + 3y as the result.

More on partial derivatives: http://www.analyzema...erivatives.html

### #3 NeoTifa

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## Re: Partial Derivatives

Posted 19 January 2011 - 06:10 PM

See, she never said that. She just threw it up on the board and did shit. Thanks.

### #4 macosxnerd101

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## Re: Partial Derivatives

Posted 19 January 2011 - 06:13 PM

No problem.

### #5 lordofduct

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## Re: Partial Derivatives

Posted 20 January 2011 - 08:37 AM

NeoTifa, on 19 January 2011 - 06:10 PM, said:

See, she never said that. She just threw it up on the board and did shit. Thanks.

It's probably because she assumed this was refresher material for everyone. It's covered toward the end of Calc1 or the beginning of Calc2 (depending on your school... my school used 1 humongous book that spanned all 3 classes. The chapters covering partial derivatives straddled the end of 1 and the beginning of 2).

I would suggest spending a weekend refreshing yourself on Calc1 and 2... Calc3 probably got you all spinning in the 3d world, vectors, normals, surfaces, etc... you probably need to get back to the standard old "this is a derivative, this is an integral" bits. Because in DiffyQ it's a lot of assumptions on the professors part that you know how to perform complex derivatives/integrals real fast and usually in your head... as if they're your multiplication tables.

What's good is DiffyQ pretty much avoids the whole 3d thing, and if you get polydimensional equations where n > 2, you don't need to graph it.

This post has been edited by lordofduct: 20 January 2011 - 08:42 AM

### #6 NeoTifa

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## Re: Partial Derivatives

Posted 20 January 2011 - 11:07 AM

Yea, thank god for that. We're only going up to touch 2nd order for a little bit towards the end of class. I honestly do not remember any of that from calc 1 or 2, and even if we did, I wouldn't remember how to do it. Just like integrating by parts, it's been over a year. >_> But seriously, I would at least remember that we did that. We did not.

### #7 macosxnerd101

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## Re: Partial Derivatives

Posted 20 January 2011 - 11:16 AM

@lordofduct: We use one book that covers Calc 1-3 as well, but I don't think we're going to hit partial derivatives this year in BC Calculus.

@NeoTifa: Integrate(u dv) = uv - Integrate(v du)

This post has been edited by macosxnerd101: 21 January 2011 - 09:41 AM
Reason for edit:: Partial derivatives, not partial fractions.

### #8 NeoTifa

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## Re: Partial Derivatives

Posted 20 January 2011 - 12:21 PM

Oh yea.

### #9 lordofduct

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## Re: Partial Derivatives

Posted 20 January 2011 - 12:49 PM

macosxnerd101, on 20 January 2011 - 11:16 AM, said:

@lordofduct: We use one book that covers Calc 1-3 as well, but I don't think we're going to hit partial fractions this year in BC Calculus.

It's somewhere in there... I can barely remember when I was in Calc class sometimes. I just remember seeing it up on the chalk board next to the goofy drawings of robbers entering a town and robbing a bank (he had this weird analogy for partial derivatives. It didn't help in explaining it, but it did help in being weird enough for me to remember the day.). I just know where it is in my text book (I still use that thing to this very day), and that's roughly the area of the book it is in.

This post has been edited by lordofduct: 20 January 2011 - 12:50 PM

### #10 NeoTifa

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## Re: Partial Derivatives

Posted 21 January 2011 - 09:39 AM

Our book is calc 1-4. >_>

### #11 macosxnerd101

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## Re: Partial Derivatives

Posted 21 January 2011 - 09:41 AM

lordofduct, on 20 January 2011 - 04:49 PM, said:

macosxnerd101, on 20 January 2011 - 11:16 AM, said:

@lordofduct: We use one book that covers Calc 1-3 as well, but I don't think we're going to hit partial fractions this year in BC Calculus.

Whoops, I meant partial derivatives, not partial fractions!

### #12 lordofduct

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## Re: Partial Derivatives

Posted 25 January 2011 - 07:22 AM

macosxnerd101, on 21 January 2011 - 09:41 AM, said:

lordofduct, on 20 January 2011 - 04:49 PM, said:

macosxnerd101, on 20 January 2011 - 11:16 AM, said:

@lordofduct: We use one book that covers Calc 1-3 as well, but I don't think we're going to hit partial fractions this year in BC Calculus.

Whoops, I meant partial derivatives, not partial fractions!

I assumed as much...

@neotifa - they broke calc into 4 sections at your school? Did they reduce the credit hour size?

At my school it was 3 classes, each 4 credit hours (the average class was 3 credit hours). I could see schools simply turning that into 4 classes, each 3 credit hours. I wonder how they group the sub topics... I bet it certainly would have made it a lot easier to consume for a lot of the students who struggled when I was in it.

### #13 NeoTifa

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## Re: Partial Derivatives

Posted 25 January 2011 - 07:47 AM

For the most part, it's 3 classes, but the 4th is usually an option or for higher mathematics, MEs, and EE's. It usually goes into more of the 3D realm than we hit in calc 3. It also hits double integrals. Really, it's kinda just a wrap up. And they're all 5 credits each I think.

### #14 lordofduct

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## Re: Partial Derivatives

Posted 25 January 2011 - 08:08 AM

What's the average credit hours at your school?

cause we just did that in calc 3

I guess Ohio just goes about it differently then here.

This post has been edited by lordofduct: 25 January 2011 - 08:10 AM

### #15 macosxnerd101

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## Re: Partial Derivatives

Posted 25 January 2011 - 08:20 AM

Yah, I thought double integrals were at the end of Calc 2, beginning of Calc 3. Unless I'm missing something, aren't they just basically:
```Integrate( Integrate(yx dx) dy) =
Integrate(1/2 yx^2 dy) =
1/4 (yx)^2

```

Just integrate for x and treat y as a constant, then integrate y and treat x as a constant?