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Villaretq0

Villaretq0

Answered question

2022-06-11

I would like to solve:
y y x y 2 = 0
In order to do so, we let y = t, and we assume x as a function of t. Now, we take derivative with respect to t from the differential equation, and obtain
d y d t x t d x d t 2 t = 0
By the chain rule, we have: d y / d t = t d x / d t. So, the above simplifies to
x = 2 t
That is, we have: x = 2 d y / d x. Thus, we obtain
y = x 2 4 + C
Now, if we want to verify the solution, it turns out that C must be zero, in other words, y = x 2 / 4 satisfies the original differential equation.
I have two questions:
1) What happens to the integration constant? That is, what is the general solution of the differential equation?
2) If we try to solve this differential equation with Mathematica, we obtain
y = C 1 x + C 1 2 ,
which has a different form from the analytical approach. How can we also produce this result analytically?

Answer & Explanation

Kamora Greer

Kamora Greer

Beginner2022-06-12Added 16 answers

You have approached the problem correctly. The reason why the arbitrary constant vanishes is because it represents a singular solution of the differential equation. The singular solution is basically like an envelope of all the solutions of the differential equation. To find the general solution, we do the following, let p = d y d x . The differential equation is
y p x p 2 = 0
Differentiating just like you did,
p p x p 2 p p = 0 p = 0
This is the part you forgot to consider.
p = C 1
where C 1 is an arbitrary constant. When we substitute this in the given differential equation, we get
y C 1 x C 1 2 = 0 y = C 1 x + C 1 2
The other possible solution is the one you obtained, i.e the singular solution
y = x 2 4

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