Author 
Topic 

uberclay
Advanced Member
Canada
159 Posts 
Posted  10/02/2007 : 23:40:37

Q: Provide the First Principles definition of dy/dx.
I've been looking through my text and cannot find a clear definition of what First Principles is. However, it does show how to differentiate using first principles, as a method (of which my wrist has suffered countless uses).
The answer I've come up with is.
Let A (x, f(x)) be any point on the curve f(x). Let P be any other point, different from A by Δx, defined as P (x + Δx, f(x + Δx)).
Therefore, the slope of the secant AP
= Δy/Δx = [f(x + Δx)  f(x)] / Δx
And the derivative (dy/dx) can be defined as
dy/dx = Δy/Δx lim(Δx>0) [f(x + Δx)  f(x)] / Δx
Am I in the ballpark? 


tkhunny
Advanced Member
USA
1001 Posts 
Posted  10/03/2007 : 08:31:46

"First Principles" appears in many fields of study and practice. Generally, it means only the most fundamental methodology possible. It is not always obvious what that means. I think your limit definition of the derivative is a good shot at it. 


uberclay
Advanced Member
Canada
159 Posts 
Posted  10/03/2007 : 13:08:04

Thanks!! 


uberclay
Advanced Member
Canada
159 Posts 
Posted  10/04/2007 : 14:53:51

I am further confounded by this as a later question asks:
For any function f(x), express dy/dx using limit notation. Give a clear explanation for your given answer.
I've revised my First Principles definition to:
Let A (x, f(x)) be the point on the curve f(x) where we want to find the slope. Let P (x + Δx, f(x + Δx) be another point, different from x of A by Δx.
The slope (m) of the secant AP is given by
Δy/Δx = [f(x + Δx)  f(x)] / [(x + Δx)  x] = [f(x + Δx)  f(x)] / Δx
As P approaches A (and Δx approaches zero), the slope (m) of the secant AP approaches the slope of the tangent to the curve f(x) at A.
lim(P>A) Slope of AP = lim(Δx>0) Δy/Δx
The derivative of f(x) is the instantaneous rate of change of y with respect to x (Δy/Δx) at any particular point on the curve f(x). Therefore the derivative (dy/dx) can be defined as
lim(Δx>0) Δy/Δx
or
lim(Δx>0) [f(x + Δx)  f(x)] / Δx
I can't seem to express this definition without limit notation so I am at a standstill.
Anyway, there's a teacher chat in 2:30 that I hope I can make. In the mean time, I'll read through the text again... and try the Google.
I'd appreciate any input. 


uberclay
Advanced Member
Canada
159 Posts 
Posted  10/04/2007 : 18:47:44

Yes David, I do. And I have used it many times. I appreciate your query.
I think all this may require is
y' = lim(Δx>0) [f(x + Δx)  f(x)] / Δx
or
f'(x) = lim(Δx>0) [f(x + Δx)  f(x)] / Δx
Where the "y'" and "f'(x)" are the limit notation asked for. 
Edited by  uberclay on 10/04/2007 18:51:27 


uberclay
Advanced Member
Canada
159 Posts 
Posted  10/04/2007 : 19:19:34

Much respect for those chat teachers. They have a lot of students eager for answers.
I'm all set. What they are looking for is
dy/dx = f'(x) f'(x) = ...
And an explination.
Good night! 
Edited by  uberclay on 10/04/2007 19:20:35 


pka
Advanced Member
USA
2731 Posts 
Posted  10/04/2007 : 19:38:38

quote: Originally posted by uberclayThe slope (m) of the secant AP is given by Δy/Δx = [f(x + Δx)  f(x)] / [(x + Δx)  x] = [f(x + Δx)  f(x)] / Δx As P approaches A (and Δx approaches zero), the slope (m) of the secant AP approaches the slope of the tangent to the curve f(x) at A. lim(P>A) Slope of AP = lim(Δx>0) Δy/Δx The derivative of f(x) is the instantaneous rate of change of y with respect to x (Δy/Δx) at any particular point on the curve f(x). Therefore the derivative (dy/dx) can be defined as
lim(Δx>0) Δy/Δx or lim(Δx>0) [f(x + Δx)  f(x)] / Δx[/b] I can't seem to express this definition without limit notation so I am at a standstill.
Oh yes yes you can! Get your self the book Creative Mathematics by H. S. Wall [University of Texas Press]. Wall gives a totally limit free approach to slope beginning on page 28. 


uberclay
Advanced Member
Canada
159 Posts 
Posted  10/05/2007 : 00:06:47

Thanks PKA. 



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