First order non-linear non-homogeneous DE problem

In summary, the differential equation is (sinx)y' - (cosx)y = 1 + C and the given solution is y = -x.cosx + sinxln(|sinx|) - Acosx + Bsinx. The student attempted to solve the equation using two methods, one using |sinx|^(-1) and the other using -|sinx|, but encountered errors. The correct solution is a linear equation on the form y(x) = \left(\int g(x)e^{\int f(x)\text{d}x}\text{d}x+K_1\right)e^{\int (-f(x))\text{d}x}.
  • #1
achacttn
7
0
Hello. I am trying to solve this problem methodically but my solution does not seem to agree with the given answer.

Homework Statement



The differential equation is:
(sinx)y' - (cosx)y = 1 + C

Homework Equations


The Attempt at a Solution



y' - (cosx/sinx)y = 1/sinx + C/sinx

When finding the integrating factor, I used:
e^-∫(cosx/sinx)dx
I was wondering if this was either
|sinx|^(-1) or -|sinx| and why? I tried solving the equation using both methods

1st method (using |sinx|^(-1))

(|sinx|^(-1))y' - cosx/((sinx)^2).y = 1/(sinx)^2 + C/(sinx)^2
LHS becomes
D[|sinx|^(-1).y] = 1/(sinx)^2 + C/(sinx)^2
Integrating both sides and multiplying by |sinx| gives
y = x/sinx + Cx/sinx + Dsinx

When using -|sinx| I get:
y = x/sinx Cx/sinx + D/sinx

The given solution is
y = -x.cosx + sinxln(|sinx|) - Acosx + Bsinx

I have no idea how to get this. Any help would be appreciated, thanks!

Edit: I see my error for using the |sinx|^(-1), I didn't integrate the right hand side properly.

I get |sinx|^(-1).y = -cotx + Ccotx + D
so
y = -xcosx + Ccosx + Dsinx, which is still not the right answer
 
Last edited:
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  • #2
It is in fact linear and on the form:
[itex] y' + f(x)y+g(x) = 0[/itex]
The solution should be as follows:
[itex] y(x) = \left(\int g(x)e^{\int f(x)\text{d}x}\text{d}x+K_1\right)e^{\int (-f(x))\text{d}x}[/itex]
 

Related to First order non-linear non-homogeneous DE problem

1. What is a first order non-linear non-homogeneous DE problem?

A first order non-linear non-homogeneous DE problem is a differential equation that involves a first derivative, contains non-linear terms, and has a non-zero constant term. This type of DE can be solved using various methods, such as separation of variables, substitution, and integrating factors.

2. How do you know if a DE problem is non-linear?

A DE problem is considered non-linear if it contains variables or functions raised to a power other than 1, or if it has products of variables or functions. In contrast, linear DE problems have variables or functions raised to the power of 1 and no products of variables or functions.

3. What is the difference between a non-homogeneous and a homogeneous DE problem?

A non-homogeneous DE problem has a non-zero constant term, while a homogeneous DE problem has a constant term of 0. In other words, a non-homogeneous DE involves an external forcing function, while a homogeneous DE does not.

4. Can a first order non-linear non-homogeneous DE problem be solved analytically?

In most cases, a first order non-linear non-homogeneous DE problem cannot be solved analytically. This means that an exact solution in the form of a function cannot be obtained. Instead, numerical methods or approximations must be used to solve the problem.

5. What are some real-world applications of first order non-linear non-homogeneous DE problems?

First order non-linear non-homogeneous DE problems can be used to model various physical, biological, and economic phenomena, such as population growth, chemical reactions, and electrical circuits. They are also commonly used in engineering and physics to study systems that involve non-linear forces and external inputs.

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