Considering this first-order linear differential equation: dy/dx+2y=0 Although I now know the correct general solution to be y=c1e−2x, I cannot figure out where I am going wrong with this apparently fallacious reasoning: dydx+2y=0 ∫dy/dx dx=∫−2y dx y=−2xy+c_1 y(2x+1)=c_1 y=c^1/(2x+1)

Gardiolo0j

Gardiolo0j

Answered question

2022-09-29

Considering this first-order linear differential equation:

d y d x + 2 y = 0
Although I now know the correct general solution to be y = c 1 e 2 x , I cannot figure out where I am going wrong with this apparently fallacious reasoning:

d y d x + 2 y = 0
d y d x d x = 2 y   d x
y = 2 x y + c 1
y ( 2 x + 1 ) = c 1
y = c 1 ( 2 x + 1 )

Answer & Explanation

Roger Clements

Roger Clements

Beginner2022-09-30Added 4 answers

Correct way to solve would be
d y d x + 2 y = 0 d y y = 2 d x d y y = 2 d x ln y = 2 x + C 1 y = e C 1 e 2 x = C e 2 x
Russell Marsh

Russell Marsh

Beginner2022-10-01Added 1 answers

The key thing here is to separate the variables such that y and dy are on the LHS whilst dx is on the RHS as given below:

d y d x = 2 y
1 y d y = 2   d x
Integrating both sides (C is a constant):

ln y = 2 x + C
y = A e 2 x
where A = e c

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