I have a first order linear differential equation (a variation on a draining mixing tank problem) wi

Cristopher Knox

Cristopher Knox

Answered question

2022-07-10

I have a first order linear differential equation (a variation on a draining mixing tank problem) with many constants, and want to separate variables to solve it.
d y d t = k 1 + k 2 y k 3 + k 4 t
y is the amount of mass in the tank at time t, and for simplicity, I've reduced various terms to constants, k 1 through k 4 .
Separation of variables is made difficult by k 1 , and I've considered an integrating factor, but think I might be missing something simple.

Answer & Explanation

Alisa Jacobs

Alisa Jacobs

Beginner2022-07-11Added 13 answers

d y d t = k 1 + k 2 y k 3 + k 4 t
For simplicity substitute u = k 3 + k 4 t
k 4 d y d u = k 1 + k 2 y u
y k 2 k 4 u y = k 1 k 4
Solve the homogeneous DE and use variation of parameter method for the inhomogeneous DE:
y k 2 k 4 u y = 0

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