Differential Equations Integrating y by x. This may be a bit of a silly questions, but when solving a differential equation by finding an integrating factor, is it possible to integrate a function of y and x by x? I understand that in multi variable calculus the y would be treated as a constant, but I am not sure why the same does not apply in differential equations. For example... For the differential equation y′+y=xy^3

teevaituinomakw

teevaituinomakw

Answered question

2022-09-17

Differential Equations Integrating y by x
This may be a bit of a silly questions, but when solving a differential equation by finding an integrating factor, is it possible to integrate a function of y and x by x? I understand that in multi variable calculus the y would be treated as a constant, but I am not sure why the same does not apply in differential equations. For example...
For the differential equation y + y = x y 3
The way to solve it would be to multiply the whole equation by 2 y 3 then solve for the integrating factor which would be e 2 x .
But I'm wondering why we even need to get rid of the y terms on the right side of the equations. Can we solve the equations as such...
y + y = x y 3
y e x + e x y = x y 3 e x
d ( y e x ) = x y 3 e x d x
I understand that this method is incorrect, but I am having trouble understanding why separation of variables are absolutely necessary in differential equations.

Answer & Explanation

Giancarlo Callahan

Giancarlo Callahan

Beginner2022-09-18Added 12 answers

Step 1
y e x + e x y = x y 3 e x
Then make it separable:
( y e x ) = x y 3 e x
d ( y e x ) y 3 e 3 x = x e 2 x d x
Step 2
And integrate:
d ( y e x ) ( y e x ) 3 = x e 2 x d x
Otherwise you can't evaluate the integral:
I = y 3 x e x d x
Because y is not a constant but a function of the variable x.

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