Northwest Kiteboarding
Forum | Classifieds | Lost & Found | CGKA | Industry | Sensors | Forecast | Spots | Seattle | Decals | RSS | Facebook

Events | Photos | Search | Register | Profile | Log in to check your messages | Log in 

Absurdly Off Topic: Any math majors/Profs out there?

 
Post new topic   Reply to topic    Northwest Kiteboarding -> Gorge / Portland / Oregon Coast
previous topic :: next topic  
Author Message
eric

Since 13 Jan 2006
1803 Posts

XTreme Poster



PostSun Jan 27, 19 1:21 pm    Absurdly Off Topic: Any math majors/Profs out there? Reply with quote

Doing a self study ODE course. Fun stuff, and for the most part smooth sailing.

I have three problems that are giving me fits, one of which is below. On-line ODE calculators do numerical analysis, whereas I need a solution done in exact form. I am many hours in, and keep ending up with a nasty integral, that seems to defy any attempts at an exact solution.

Here is the original problem from Stroud DE page 39:

y(xy+1) +x(1+xy+x^2y^2)(dy/dx) = 0 Let y = v/x

"Transformations. Make the given substitution and use methods of first order homogeneous equations"

I know how to do substitutions, Bernoulli, and use an Integrating Factor. We are not supposed to apply any numerical/ La Place method at this point. I have been treating this as a Separable DE post substitution. I feel like I am not seeing something obvious.

Long shot to post this here, but why not!?

Thanks,
Eric

Last edited by eric on Sun Jan 27, 19 1:54 pm; edited 1 time in total

View user's profile Send private message
jasonq

Since 12 Jan 2009
242 Posts
hood river
Stoked



PostSun Jan 27, 19 1:44 pm     Reply with quote

i am neither, and a many, many years away from DE classes.

that said, mathematica can almost certainly deal with the problem using symbolic math, not numerical methods (unless you want it to). I rely on mathematica so much, i am not sure i can do addition by hand anymore.

a mathematica license is $$$ but a little-ish known fact is that the Raspbian OS for the Raspberry Pi includes a free copy of mathematica. For about $50 total for a RB Pi 3B+ and an SD Card you can run it to your heart's content.

I think but am not sure there is a way to get wolfram alpha to do it to via a web browser.

View user's profile Send private message
wylieflyote

Since 30 Jun 2006
1634 Posts
Puget Sound & Wa. Coast
XTreme Poster



PostSun Jan 27, 19 4:55 pm     Reply with quote

Are boardshorts over a wetsuit still cool?
_________________
CGKA Member
-------
Kip Wylie

View user's profile Send private message
consumer

Since 28 Nov 2010
406 Posts
banned
Obsessed



PostSun Jan 27, 19 6:56 pm     Reply with quote

y(xy+1) +x(1+xy+x^2y^2)(dy/dx) = 0 Let y = v/x

is v a variable?

if not then dy/dx = -v/x^2

v/x(x*v/x+1)+x(1+x*v/x+x^2(v/x)^2)*(-v/x^2)=0

v/x(v+1)+x(1+v+v^2)(-v/x^2)=0

v/x(v+1)+(1+v+v^2)(-v/x)=0

[v(v+1)/x-v(1+v+v^2)]/x = 0

[v(v+1-1-v-v^2)]/x=0

v^3/x=0

um lol v=0 or x = infinity

obviously i'm missing something too.

if v is a variable then dy/dx = -v/x2 + dv/xdx

and is now out of my pay grade. good luck.

View user's profile Send private message
eric

Since 13 Jan 2006
1803 Posts

XTreme Poster



PostSun Jan 27, 19 7:17 pm     Reply with quote

Yeah, V is a variable.

View user's profile Send private message
1surfer

Since 20 Apr 2010
354 Posts
'Ualapu'e / High Rockies
Obsessed



PostFri Feb 01, 19 2:22 pm    XXOO Reply with quote

XXX OOO = hugs and kisses
_________________
Ozone Reo's. C&k Beach Boy Service, Windsurfing Waikiki, Hawaiian Windriders, NSRN.

View user's profile Send private message Visit poster's website
scottm

Since 12 Jun 2006
108 Posts

Stoked



PostSat Feb 02, 19 1:51 pm     Reply with quote

Board shorts over a full wetsuit never were cool, and never will be cool. And failing to define V as a third variable, makes your professor, somebody who would wear board shorts over a wetsuit. But that was probably one of the unstated assumptions...

View user's profile Send private message
undertow

Since 15 Feb 2008
371 Posts
BeaversBurg
Obsessed



PostWed Feb 06, 19 2:20 pm     Reply with quote

Fallout 76 is a great game!!!!!

View user's profile Send private message
Post new topic   Reply to topic    Northwest Kiteboarding -> Gorge / Portland / Oregon Coast All times are GMT - 8 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum