Separable but not exact equation (xlny+xy)+(ylnx+xy)y'=0

Elise Kelley

Elise Kelley

Answered question

2022-10-28

Separable but not exact equation
In class, my professor stated that all separable equations are exact, and we even proved it for homework, but I think I found an equation that is separable but not exact:
( x ln y + x y ) + ( y ln x + x y ) y = 0
My Work:
M = x ln y + x y M y = x y + x N = y ln x + x y N x = y x + y M y N x
But
x ( ln y + y ) + y × y × ( ln x + x ) = 0 x ( ln y + y ) = y × y × ( ln x + x ) x ln x + x = y × y ln y + y
Separated
So whats up? Am I doing something wrong?

Answer & Explanation

Momellaxi

Momellaxi

Beginner2022-10-29Added 14 answers

The correct statement is that a separable differential equation in the form
a ( x ) d x + b ( y ) d y = 0
is exact. But if you multiply an exact differential equation by some function of x and/or y, it generally will cease to be exact. So, for example, x a ( x ) d x + x b ( y ) d y = 0 will not be exact, though it is still separable.

Do you have a similar question?

Recalculate according to your conditions!

New Questions in Differential Equations

Ask your question.
Get an expert answer.

Let our experts help you. Answer in as fast as 15 minutes.

Didn't find what you were looking for?