Who's good at calc?

[Platypus]

Second order partial derivatives, mainly finding the slope of a level curve with two variables. (dy/dx) = -fx/fy

I have this equation:

f(x,y) = x^2 + xy + y^3 = 8
x=0


So then:

fx = 2x +y
fy = x+3y^2

if my partial derivative math is correct thus making the equation:

-2x+y / x + 3y^2 or -2/3y with some cancelling.

Plugging 0 for x yields 2 for y, leaving the equation to be -2/6 or -1/3
However the book gets -1/6

Where did I go wrong? Or am I?

 

[spanky]

dude.... that hurts my eyes

 

[Ness]

I failed calc.



Twice.


I don't think you want my help. haha.

 

[Oscar1613]

it should be -(2x + y)/(x + 3y^2), which doesnt reduce... put 0 and 2 into that and you do get -1/6

 

[Heisenberg]

You can't cancel x and y terms separately like that. I don't think that will reduce at all. Just put in the numbers in and don't simplify.

Edit: Damn. Too slow.

 

[Platypus]

Originally posted by: Oscar1613
it should be -(2x + y)/(x + 3y^2), which doesnt reduce... put 0 and 2 into that and you do get -1/6

Ah, I see thanks a lot.

 

[kevinthenerd]

Originally posted by: Oscar1613
it should be -(2x + y)/(x + 3y^2), which doesnt reduce... put 0 and 2 into that and you do get -1/6

Yeah, I noticed a simple algebraic error you made.

-(2x + y) != -2x + y

-(2x + y) = -2x - y


Get some sleep. If you're this far in math, such simple errors are definitely from a lack of sleep. I've been there.


(for you non programmers, I use != to mean "not equal to")



EDIT:

Originally posted by: Heisenberg
Edit: Damn. Too slow.

 

[Platypus]

Originally posted by: kevinthenerd

Originally posted by: Oscar1613
it should be -(2x + y)/(x + 3y^2), which doesnt reduce... put 0 and 2 into that and you do get -1/6

Yeah, I noticed a simple algebraic error you made.

-(2x + y) != -2x + y

-(2x + y) = -2x - y


Get some sleep. If you're this far in math, such simple errors are definitely from a lack of sleep. I've been there.


(for you non programmers, I use != to mean "not equal to")



EDIT:

Originally posted by: Heisenberg
Edit: Damn. Too slow.

Yeah, thanks a lot guys. I am at work right now so sleep isn't very close. I am just so damn fried from classes/work all day that I totally missed it. I feel foolish now, thanks again.

 

[Koing]

I thought that bit was iffy myself .

Hope I get this much help when I need it when I start class's in 4 weeks.

Once again ATOT has proven it's worth (if there ever was a doubt! :Q)

Koing

 

[raptor13]

This isn't a slam or anything because I do things like this all the time... but isn't it funny how you're doing some somewhat complex math (that would be the partial differentiating) yet you mess up on something as simple as the Distributive Rule?


I always get a kick out of that. Except when it happens to me. And then it sucks.

 

[kevinthenerd]

Originally posted by: raptor13
This isn't a slam or anything because I do things like this all the time... but isn't it funny how you're doing some somewhat complex math (that would be the partial differentiating) yet you mess up on something as simple as the Distributive Rule?


I always get a kick out of that. Except when it happens to me. And then it sucks.

I do that all the time, guys. It's like programming... I always spend 80% of my time "debugging" the simple pieces of complex assignments. If only homework was object-oriented...


Hey, that gives me an idea.