For example, if I have the system, y &#x2032; </msup> + y = 3 x

Izabella Ponce

Izabella Ponce

Answered question

2022-06-20

For example, if I have the system,
y + y = 3 x y y = x
I could then use elimination to minus the top equation from the bottom one to get,
2 y = 2 x y = x
Which is obviously wrong as then, 1 + x = 3 x which is wrong.
So why are you not able to use elimination in solving a system of first order differential equations?

Answer & Explanation

Blaine Foster

Blaine Foster

Beginner2022-06-21Added 33 answers

The equivalence is really between these systems of equations
| y + y = 3 x y y = x | | y + y = 3 x y = x | | y y = x y = x |
so your error is that you dropped one of the original equations, which leads to a larger set of solutions than it should.
However the above equivalence is holding:
The general solution of y + y = 3 x is y ( x ) = c e x + 3 x 3.
The general solution of y y = x is y ( x ) = c e x + 3 x 3.
The general solution of y y = x is y ( x ) = c e x x 1.
So the first system has no solution. As does the second system. And as does the third system. They all have the emtpy set as set of solutions.

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