So I have the differential equation ( y 2 </msup> + x y ) d x + (

Willow Pratt

Willow Pratt

Answered question

2022-07-01

So I have the differential equation
( y 2 + x y ) d x + ( 3 x y + x 2 ) d y = 0. I am asked to solve this ODE by using the substitution v = y x but I run into some issues if I do that.
If v = y x then y ( x ) = v + x v
I then divided every term by x 2 to get
( y 2 x 2 + y x ) + ( 3 y x + 1 ) ( d y d x ) = 0
If I substitute, I then get,
( v 2 + v ) + ( 3 v + 1 ) ( v + x v ) = 0
Now I run into the problem where I can't make this into a separable equation...
I know that this is an exact equation and I can solve it if I use an integration factor u ( x ) = y but I am asked to solve this question using the substitution v = y x . Even if I muliply each term by the integration factor, things become even worse...
Any guidance would be appreciated. Thanks.

Answer & Explanation

Jaelynn Cuevas

Jaelynn Cuevas

Beginner2022-07-02Added 16 answers

( y 2 + x y ) d x + ( 3 x y + x 2 ) d y = 0 d y d x = ( y 2 + x y ) ( 3 x y + x 2 ) put y = v x y = v + x v x d v d x = v 2 v 3 v + 1 v = 2 v ( 2 v + 1 ) 3 v + 1 3 v + 1 2 v ( 2 v + 1 ) d v = d x x ( 2 v + 1 ) + ( v ) 2 v ( 2 v + 1 ) d v = d x x 1 2 v d v 1 2 1 2 v + 1 = log x + log c 1 2 log v 1 4 log ( 2 v + 1 ) = log c x 1 v . ( 2 v + 1 ) 1 / 4 = c x just put the value of  v = y x .
lilmoore11p8

lilmoore11p8

Beginner2022-07-03Added 6 answers

In the equation ( y 2 + x y ) P d x + ( 3 x y + x 2 ) Q d y = 0 , the function P and Q are homogeneous with the same degree d (grade 2 in this case). By a well-kown property, the substitution y = v x transforms the given equation in other one with separate variables.
Proof
y = v x P ( x , y ) = P ( x , v x ) , Q ( x , y ) = Q ( x , v x ) .
As P and Q are homogeneous with degree d:
P ( x , v x ) = P ( 1 x , v x ) = x d P ( 1 , v ) = x d R ( v ) ,
Q ( x , v x ) = Q ( 1 x , v x ) = x d Q ( 1 , v ) = x d S ( v ) .
We get ( R ( v ) + v S ( v ) ) d x + x S ( v ) d v = 0  (separate variables)

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