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2013-09-21 3:02 PM

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Elite
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Subject: Math geeks - help needed
I have a graph with the following points defined:

(0,0)
(10,2)

The tangent at the first point is horizontal. How do I find a 3rd degree polynomial equation for the line? I'm trying to create a particular shape and while I don't understand the subject matter quite yet, it's been determined by folks smarter than I am that the segment of a 3rd degree polynomial is the right choice. I'd rather have the process than just the answer, since I have another of these to figure out.

I'm guessing there's some calc involved if I remember one of the points of derivatives correctly.


2013-09-21 3:12 PM
in reply to: dgunthert

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Regular
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Subject: RE: Math geeks - help needed
2013-09-21 5:29 PM
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Master
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Somewhere on the Tennessee River
Subject: RE: Math geeks - help needed

Originally posted by dgunthert I have a graph with the following points defined: (0,0) (10,2) The tangent at the first point is horizontal. How do I find a 3rd degree polynomial equation for the line? I'm trying to create a particular shape and while I don't understand the subject matter quite yet, it's been determined by folks smarter than I am that the segment of a 3rd degree polynomial is the right choice. I'd rather have the process than just the answer, since I have another of these to figure out. I'm guessing there's some calc involved if I remember one of the points of derivatives correctly.

 

If you had a third point it is easy to model with simple algebra.    As it is with  points on the graph and the first derivative of first point it is a differential equations problem.  What is the particular shape or curve that you wish to model?  Also, do you know the slope of the function at the second point?



Edited by MadMathemagician 2013-09-21 5:30 PM
2013-09-21 5:51 PM
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Master
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Subject: RE: Math geeks - help needed

y= (5/4)x^3, with only a little understanding of calculus needed.

Start with the generic formula y = ax^3 + bx^2 + cx + d.    There is enough information given  to generate this answer.



Edited by MadMathemagician 2013-09-21 5:54 PM
2013-09-21 6:04 PM
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Master
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Somewhere on the Tennessee River
Subject: RE: Math geeks - help needed

We see d = 0 because given the point (0,0)  we have 0 = a(0)^3 + b(0)^2 +c(0) +d

this gives 0 = 0 + 0 + 0 + d,  implying d = 0.



Edited by MadMathemagician 2013-09-21 6:05 PM
2013-09-21 6:10 PM
in reply to: MadMathemagician

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Master
1970
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Somewhere on the Tennessee River
Subject: RE: Math geeks - help needed

the derivative of our generic function is y' = 3ax^2 + 2bx + c.

The slope of the first derivation is zero so at x = 0 then y' = 0.

0 = 3a(0)^2 + 2b(0) + c  implies that also c = 0.

So our generic function is now defined as y = ax^3 + bx^2.    



2013-09-21 6:33 PM
in reply to: MadMathemagician

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Master
1970
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Somewhere on the Tennessee River
Subject: RE: Math geeks - help needed

Since we want as smooth a curve as possible we make an assumption that b = 0.   This, because with two terms in our equation we get an oscillation that degrades "smooth."   With this assumption we can use the point (10,2)  and plug into y = ax^3 to solve for a.

This satisfies all given conditions.

2013-09-21 6:35 PM
in reply to: MadMathemagician

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Master
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Subject: RE: Math geeks - help needed

Bythe way, my fee for tutoring is $50/hr.      Buy me a beer and we will call it even....   {#emotions_dlg.cool}

 

2013-09-21 6:48 PM
in reply to: MadMathemagician

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Subject: RE: Math geeks - help needed
my head hurts just reading this thread...I agree with Switch
2013-09-21 7:03 PM
in reply to: 0

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Master
1970
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Subject: RE: Math geeks - help needed

P.S.   I, by no stretch of anyone's imagination, can be called a  "geek."    

 

P.S.S     Lately I'm partial to anything Blue Moon.   Any of the seasonal brews will do nicely....   Wink



Edited by MadMathemagician 2013-09-21 7:04 PM
2013-09-21 8:43 PM
in reply to: MadMathemagician

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Supersonicus Idioticus
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Subject: RE: Math geeks - help needed
Ok, so I understand you are a PhD in math, so you are the authority.

However, should we be asking if a 3rd degree polynomial is appropriate?

We have two points, so a line between the two is the simplest "line of best fit." Since the tangent at the origin is zero, then joining these two points needs to be done with at least a quadratic (ax^2+bx+c | c=0)

Joining these two points could be done with a 4th order, 8th order, or 21st order polynomial... so why are we picking the 3rd order?

To the OP: If you want a 2nd order equation that solves your needs, how about:

f(x) = 0.02 * x^2

??


2013-09-21 8:53 PM
in reply to: So Fresh So Clean

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Master
1970
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Subject: RE: Math geeks - help needed

Originally posted by So Fresh So Clean Ok, so I understand you are a PhD in math, so you are the authority. However, should we be asking if a 3rd degree polynomial is appropriate? We have two points, so a line between the two is the simplest "line of best fit." Since the tangent at the origin is zero, then joining these two points needs to be done with at least a quadratic (ax^2+bx+c | c=0) Joining these two points could be done with a 4th order, 8th order, or 21st order polynomial... so why are we picking the 3rd order? To the OP: If you want a 2nd order equation that solves your needs, how about: f(x) = 0.02 * x^2 ??

You are correct in that we can pick an equation of any degree and fit according.   Not knowing any of the points in between the two given means we have no clue as to how accurate any equation of any degree will be.   Therefore any degree we wish to use will be just as good as another.

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