Problem: Solve the following differential equations. x d y </mro

Jaiden Bowman

Jaiden Bowman

Answered question

2022-04-10

Problem:
Solve the following differential equations.
x d y d x + y = y 2
Answer:
To solve this equation, we reduce it to a linear differential equation with the substitution v = y 3 .
x y 2 d y d x + y 3 = 1 d v d x = 3 y 2 d y d x 3 x y 2 d y d x + 3 y 3 = 3 d v d x + 3 v = 3
Now we have a first order linear differential equation. To solve it, we use the integrating factor I = e P ( x ) . In this case, we have P(x)=3.
I ( x ) = e 3 x e 3 x d v d x + 3 e 3 x v = 3 e 3 x D ( e 3 x v ) = 3 e 3 x e 3 x v = e 3 x + C v = 1 + e 3 x y 3 = 1 + e 3 x
Now to check my answer.
3 y 2 d y d x = 3 C e 3 x y 2 d y d x = C e 3 x d y d x = C e 3 x y 2 x d y d x + y = x ( C e 3 x y 2 ) + ( 1 + e 3 x ) 1 3
I cannot seem to get my answer to check. Where did I go wrong?
Here is my second attempt to solve the problem:
To solve this equation, we reduce it to a linear differential equation with the substitution v = y 3 .
x y 2 d y d x + y 3 = 1 d v d x = 3 y 2 d y d x 3 x y 2 d y d x + 3 y 3 = 3 x d v d x + 3 v = 3 d v d x + 3 x 1 v = 3 x 1
Now we have a first order linear differential equation. To solve it, we use the integrating factor I = e P ( x ) . In this case, we have P ( x ) = 3 x 1 .
I = e 3 x 1 d x = e 3 ln | x | I = 3 x 3 x d v d x + 9 v = 9
Now, I want to write:
D ( 3 x v ) = 9
but that is wrong. What did I do wrong?
Here is my third attempt to solve the problem. Last time, I made a mistake in finding the integrating factor.
To solve this equation, we reduce it to a linear differential equation with the substitution v = y 3
x y 2 d y d x + y 3 = 1 d v d x = 3 y 2 d y d x 3 x y 2 d y d x + 3 y 3 = 3 x d v d x + 3 v = 3 d v d x + 3 x 1 v = 3 x 1
Now we have a first order linear differential equation. To solve it, we use the integrating factor I = e P ( x ) . In this case, we have P ( x ) = 3 x 1 .
I = e 3 x 1 d x = e 3 ln | x | I = x 3 x 3 d v d x + 3 x 2 v = 3 x 2 D ( x 3 v ) = x 3 + C x 3 v = x 3 + C v = C x 3 + 1 y 3 = C x 3 + 1
Do I have it right now?

Answer & Explanation

stafninumfu1tf

stafninumfu1tf

Beginner2022-04-11Added 18 answers

x y 2 y + y 3 = 1
or with u := y 3
x u + 3 u = 3.
We have the particular solution u p = 1, and the homogeneous equation is separable, giving
u h = c x 3 .
Finally,
y = c x 3 + 1 3 .
Check:
x y + y = x 3 c x 4 3 ( c x 3 + 1 ) 2 / 3 + c x 3 + 1 3 = 3 c x 3 + 3 x 3 + 1 ( c x 3 + 1 ) 2 / 3 = 1 y 2 .

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