Why Is the Solution to dy/dx = -x/y Expressed as y^2 + x^2 = c?

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Discussion Overview

The discussion centers on the differential equation dy/dx = -x/y and its general solution expressed as y^2 + x^2 = c. Participants explore the reasoning behind this representation compared to alternative forms, such as y = -x^2 + c, and the implications of separating variables in differential equations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the general solution y^2 + x^2 = c is a more natural way to describe the curve, which is a circle, rather than expressing it as y = -x^2 + c.
  • Others question the necessity of separating x and y, suggesting that differential equations can yield solutions that are not functions.
  • A participant points out that y = -x^2 + c does not represent the same solution as y^2 + x^2 = c, emphasizing that the latter includes both positive and negative roots.
  • There is a reiteration of the method of solving the differential equation, including steps that lead to the general solution.

Areas of Agreement / Disagreement

Participants express differing views on the representation of the solution, with some agreeing that y^2 + x^2 = c is preferable, while others emphasize the limitations of alternative forms. The discussion remains unresolved regarding the necessity of separating variables in this context.

Contextual Notes

Some assumptions about the nature of solutions to differential equations and the implications of expressing solutions in different forms are not fully explored, leading to potential misunderstandings about the completeness of the solutions.

fran1942
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Hello, regarding the differential equation: "dy/dx = -x/y"
The general solution is y^2+x^2 = c.

I am wondering why it is expressed this way instead of "y=-x^2+c" ?
I thought you had to separate the x and y to opposite sides of the equation ?

Thanks for any help.
 
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hello fran1942! :smile:

(you mean y = √(c - x2) :wink:)
fran1942 said:
… I thought you had to separate the x and y to opposite sides of the equation ?

no, there's nothing special about y …

the answer is a curve (in this case, a circle), and x2 + y2 = c is a more natural way of describing a curve :wink:
 
fran1942 said:
Hello, regarding the differential equation: "dy/dx = -x/y"
The general solution is y^2+x^2 = c.

I am wondering why it is expressed this way instead of "y=-x^2+c" ?
I thought you had to separate the x and y to opposite sides of the equation ?

Thanks for any help.

dy/dx = -x/y
y*dy = -x*dx
y²/2= -x²/2 +C
y² = -x² +c (c=2C)
y²+x² = c
 
fran1942 said:
Hello, regarding the differential equation: "dy/dx = -x/y"
The general solution is y^2+x^2 = c.

I am wondering why it is expressed this way instead of "y=-x^2+c" ?
Well, it wouldn't be expressed that way because those are not at all the same!
I presume you meant [itex]y= \sqrt{c- x^2}[/itex]. The difficulty with that is that it is only "half" of the solution- the other half would be [itex]y= -\sqrt{c- x^2}[/itex].

I thought you had to separate the x and y to opposite sides of the equation ?

Thanks for any help.
No, differential equations often have solutions that are not functions.
 

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