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bobmerhebi
Nov16-08, 02:47 PM
1. The problem statement, all variables and given/known data

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

2. Relevant equations

y = ux

3. The attempt at a solution

y = ux implies dy = udx + xdu

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

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

so (1/x).u\sqrt{}udx + (1 + \sqrt{}u)du = 0

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

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

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

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

so ln y - 2\sqrt{}x/\sqrt{}y = 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 plz. my process is right isn't it? how should i continue?

HallsofIvy
Nov16-08, 03:57 PM
The rest is just basic algebra!
From ln|y| 2\sqrt{x}/\sqrt{y}= c1, 2\sqrt{x}/sqrt{y}= ln|-|c1,
2\sqrt{x}= (ln|y|- c1)\sqrt{y}. Squaring 4x= (ln y- c1)^2y.

bobmerhebi
Nov16-08, 04:13 PM
thank u Sir for the help.

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

thx again