Separable equations: 1st order DE

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SUMMARY

The discussion focuses on solving the first-order differential equation dy/dx = (4x - x^3) / (4 + y^3). Participants clarify the process of separating variables, emphasizing that when rewriting the equation, the term (4 + y^3)dy is correctly placed on the left-hand side, while the y terms transition to the numerator. This method adheres to the mathematical principle that allows for the manipulation of fractions across equations, ensuring that the separation of variables is valid and consistent.

PREREQUISITES
  • Understanding of first-order differential equations
  • Familiarity with the separation of variables technique
  • Knowledge of algebraic manipulation of fractions
  • Basic calculus concepts, including derivatives
NEXT STEPS
  • Study the method of separation of variables in differential equations
  • Explore the implications of manipulating fractions in algebra
  • Learn about the existence and uniqueness theorems for first-order DEs
  • Practice solving various first-order differential equations
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Students studying calculus, particularly those focusing on differential equations, as well as educators seeking to clarify the separation of variables method in first-order DEs.

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Homework Statement


solve the quation dy/dx = (4x - x^3) / (4 + y^3)
the first thing the book does is rewrite the equation as:

(4+y^3)dy = (4x-x^3)dx

and i understand that they are 1st separating it out... BUT shouldn't it be (1 / (4+y^3))dy?

How can they dissmiss the fact that the y terms are in the denominator?
On every problem, the fact that terms are under denominator gets ignored --why?

Homework Equations


thanks!


The Attempt at a Solution


 
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[tex]\frac{dy}{dx} = \frac{4x - x^3}{4 + y^3}[/tex]

[tex]\times (4 + y^3)[/tex]


[tex](4 + y^3) \frac{dy}{dx} = 4x - x^3[/tex]


Now separate the variables.
 
jenzao said:
How can they dissmiss the fact that the y terms are in the denominator?

Hi jenzao! :smile:

The y terms are in the denominator on the RHS,

but when you move them over to the LHS, they must go on top.

(and the dx on the bottom of the LHS must go on the top of the RHS for the same reason)

Technically, that's because if A/B = C/D, then AD = BC. :smile:
 

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