Very very short question on second derivative

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Discussion Overview

The discussion revolves around the process of finding the second derivative of a circle defined by the equation 9x² + y² = 9 at a specific point (0, 3) using implicit differentiation. Participants explore the implications of differentiating geometric equations and the correct application of differentiation techniques.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the meaning of finding the second derivative of a circle, suggesting it may not make sense to differentiate geometric objects.
  • Another participant asserts that the equation describes an ellipse rather than a circle, which may influence the differentiation approach.
  • There is a discussion about the correct form of the first derivative, with some participants indicating that the first derivative must be solved algebraically.
  • Concerns are raised about the application of the product rule during the differentiation process, with one participant noting a potential oversight in the calculations.
  • Participants discuss the need to substitute coordinates into the derived equations to find specific values for the derivatives.
  • One participant expresses understanding of the process of finding y' and y'' through implicit differentiation and substitution.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the geometric object (circle vs. ellipse) and the implications for differentiation. There is no consensus on the correct approach to finding the second derivative, as various methods and interpretations are presented.

Contextual Notes

Limitations include potential misunderstandings about the nature of the geometric equation and the application of differentiation rules, such as the product rule. The discussion also reflects uncertainty regarding the correct interpretation of the derivatives in the context of the given equation.

CookieSalesman
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What does it mean when I have to find the second derivative of a circle at a given point? (Implicit diffing)

In specifics, the equation is 9x2 +y2 =9
At the point (0,3)
You don't really need the rest at all, but it was just my process.

This seems to make no sense.
first D'v 18x+2yy'=0
Second derivative gives 18+y''=0
Doesn't it?
How am I supposed to substitute coordinates?
 
Last edited:
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CookieSalesman said:
What does it mean when I have to find the second derivative of a circle at a given point? (Implicit diffing)

In specifics, the equation is 9x2 +y2 =9
At the point (0,3)

This seems to make no sense.
first D'v 18x+2yy'=0
That equation is not the first derivative. You need to solve for y' algebraically.
CookieSalesman said:
Second derivative gives 18+y''=0
Doesn't it?
No. When you have isolated y', differentiate it to get y''. Then, to find the value of y'' at (0, 3), substitute these coordinates in your formula.
CookieSalesman said:
How am I supposed to substitute coordinates?
 
CookieSalesman said:
What does it mean when I have to find the second derivative of a circle at a given point? (Implicit diffing)
Strictly speaking, it doesn't mean anything! You do not take the derivative of geometric objects, you take the derivative of functions. What you are asking about the relation describing the circle.
In specifics, the equation is 9x2 +y2 =9
At the point (0,3)
That's not a circle, it's an ellipse.
This seems to make no sense.
first D'v 18x+2yy'=0
Second derivative gives 18+y''=0
Doesn't it?
How am I supposed to substitute coordinates?
Using "implicit differentiation" as you say, you get 18x+ 2yy'= 0 where y' is the derivative with respect to x.
Doing that again you do NOT get 18+ y''= 0. You forgot to use the product rule on 2yy'. Implicit differentiation gives, rather,
 
Last edited by a moderator:
Okay, so if I differentiate once implicitly, I get 18+2yy'
Then, algebra, so that it's -9/y = y'
But if I differentiate that, then I have y prime and y double prime...
Oh, so I can find y prime from the earlier equation, then resubsitute?

Thanks.
 
CookieSalesman said:
Okay, so if I differentiate once implicitly, I get 18+2yy'
Then, algebra, so that it's -9/y = y'
But if I differentiate that, then I have y prime and y double prime...
Oh, so I can find y prime from the earlier equation, then resubsitute?

Thanks.
That's the idea.
 

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