Help nedded to solve a D.E. using substitution

In summary, a substitution method is used to solve the differential equation -ydx + (x + \sqrt{}xy)dy = 0. The solution involves manipulating the equation to get to the form of ln y - 2\sqrt{}x/\sqrt{}y = c1, which can then be simplified to 4x = y(ln|y| - c)2. The remaining steps involve basic algebra.
  • #1
bobmerhebi
38
0

Homework Statement



Use the appropriate substitution to solve the following D.E.: -ydx + (x + [tex]\sqrt{}xy[/tex])dy = 0

Homework Equations



y = ux

The Attempt at a Solution



y = ux implies dy = udx + xdu

so -xudx + (x + x[tex]\sqrt{}u[/tex])(udx + xdu) =0

we then get after some simplificaion: xu[tex]\sqrt{}u[/tex] dx + x2 (1 + [tex]\sqrt{}u[/tex])du = 0

so (1/x).u[tex]\sqrt{}u[/tex]dx + (1 + [tex]\sqrt{}u[/tex])du = 0

hence dx/x + du/(u[tex]\sqrt{}u[/tex]) + du/u = 0

now we have after integrating: lnx + lnu - 2/[tex]\sqrt{}u[/tex] = c1

substituting bk u= y/x we have: ln x + ln (y/x) - 2[tex]\sqrt{}x[/tex]/[tex]\sqrt{}y[/tex] = c1

ln x + ln y - lnx - 2[tex]\sqrt{}x[/tex]/[tex]\sqrt{}y[/tex] = c1

so ln y - 2[tex]\sqrt{}x[/tex]/[tex]\sqrt{}y[/tex] = c1

here i got stuck. i couldn;t continue although i know that the answer should be : 4x = y(ln|y| - c)2

need help in this please. my process is right isn't it? how should i continue?
 
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  • #2
The rest is just basic algebra!
From [itex]ln|y| 2\sqrt{x}/\sqrt{y}= c1[/itex], [itex]2\sqrt{x}/sqrt{y}= ln|-|c1[/itex],
[itex]2\sqrt{x}= (ln|y|- c1)\sqrt{y}[/itex]. Squaring [itex]4x= (ln y- c1)^2y[/itex].
 
  • #3
thank u Sir for the help.

I feel like a dum after it appeared to be that apparent.

thx again
 

Related to Help nedded to solve a D.E. using substitution

1. What is substitution in the context of solving a differential equation (D.E.)?

Substitution is a technique used in solving differential equations that involves replacing a variable with another expression in order to simplify the equation and make it easier to solve.

2. How do I know when to use substitution in solving a D.E.?

You can use substitution when the differential equation involves a variable that can be expressed in terms of another variable or when the equation has an expression that can be substituted with another expression to simplify the equation.

3. What are the steps to solve a D.E. using substitution?

The steps to solve a D.E. using substitution are:
1. Identify the variable that can be substituted
2. Determine the appropriate substitution expression
3. Substitute the expression into the D.E.
4. Simplify the equation
5. Solve the simplified equation
6. Substitute the value back into the original variable to find the final solution.

4. Can I use substitution to solve any type of D.E.?

No, substitution is not always a viable method for solving differential equations. It is most effective when solving separable equations or equations that can be reduced to separable form.

5. Are there any common mistakes to avoid when using substitution to solve a D.E.?

Yes, some common mistakes to avoid when using substitution to solve a D.E. include:
- Choosing an incorrect substitution expression
- Making errors in algebraic simplification
- Forgetting to substitute the value back into the original variable
- Forgetting to check the solution for extraneous solutions.

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