What is a solution to the differential equation dy/dx=x+y/x−y?

basaltico00

basaltico00

Answered question

2022-09-23

What is a solution to the differential equation d y d x = x + y x - y ?

Answer & Explanation

Jamari Morgan

Jamari Morgan

Beginner2022-09-24Added 10 answers

d y d x = x + y x - y
this is first order linear and homogeneous in the sense that when written in the form d y d x = f ( x , y ) then
f ( k x , k y ) = f ( x , y )
so we re-write it as d y d x = 1 + y x 1 - y x
in order to make the standard sub v ( x ) = y ( x ) x
because y=vx, then y = v x + v
so we have
v x + v = 1 + v 1 - v and we may as well now separate it out
v x = - v + 1 + v 1 - v
= - v ( 1 - v ) + 1 + v 1 - v
v x = v 2 + 1 1 - v
v - 1 v 2 + 1 v = - 1 x
  v - 1 v 2 + 1 v   d x = -   1 x   d x
looks absolutely horrible
  v - 1 v 2 + 1   d v = -   1 x   d x
  v v 2 + 1 - 1 v 2 + 1   d v = -   1 x   d x
1 2 ln ( v 2 + 1 ) - arctan v = - ln x + C
1 2 ln ( ( y x ) 2 + 1 ) - arctan ( y x ) = - ln x + C

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