A differential equation with homogeneous coefficients: (x+y)dx−(x−y)dy=0.

cousinhaui

cousinhaui

Answered question

2022-10-26

A differential equation with homogeneous coefficients: ( x + y ) d x ( x y ) d y = 0
Functions P ( x ) = x + y and Q ( x ) = x y are both homoegeneous functions of order 1. So I can use substitusion:
y = u x , d y = u d x + x d u
(because it will lead to a differential equation in which the variables are separable)
So after this substitution I'm obtaining result:
( u + u x ) d x ( x u x ) ( u d x + x d u ) = 0
After simplification I have:
x 2 ( u 1 ) d u + x ( 1 + u 2 ) d x = 0
Now I'm dividing both sides by x 2 ( 1 + u 2 ) and I'm obtaining:
u 1 1 + u 2 d u + d x x = 0
Next, because:
u 1 1 + u 2 d u = 1 2 ln | u 2 + 1 | arctan ( u ) + C
and:
d x x = ln | x | + C
I have a solution:
1 2 ln | u 2 + 1 | arctan ( u ) + ln | x | = C
1 2 ln | y 2 x 2 + 1 | arctan ( y x ) + ln | x | = C
After simplification, I can obtain result:
1 2 ln ( x 2 + y 2 ) arctan ( y x ) = C
In book from this exercise from there is a little diffrent answer :
arctan ( y x ) 1 2 ln ( x 2 + y 2 ) = C
Is my solution incorrect? I will be grateful for an explanation

Answer & Explanation

fitte8b

fitte8b

Beginner2022-10-27Added 12 answers

Both solutions are correct. Note that C is an arbitrary constant. Therefore, one may define C = k on your solution:
1 2 ln ( x 2 + y 2 ) arctan ( y x ) = k
Multiplying both sides by 1, we obtain the same solution the book obtained:
arctan ( y x ) 1 2 ln ( x 2 + y 2 ) = k

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