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MathGroup Archive 2001

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Re: help me plz

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32163] Re: [mg32149] help me plz
  • From: BobHanlon at aol.com
  • Date: Mon, 31 Dec 2001 02:24:21 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 12/29/01 6:31:44 PM, samer_ghamrawui at hotmail.com writes:

>i want to solve this equation
>
>y"*y'+2y"'=0
>
>y(0)=0
>y'(0)=0
>y'(infini)=1
>

Off[Solve::ifun];

Clear[y];

eqns = {y''[x]*y'[x]+2y'''[x]==0, y[0]==0};

soln = DSolve[eqns, y[x], x]//Flatten

{y[x] -> 4*Log[Cosh[(x*Sqrt[C[1]])/
       2]]}

The condition, y'[0] == 0 is redundant

D[y[x] /. soln, x] == 0  /. x -> 0

True


Solve[Simplify[
      D[y[x] /. soln, x]  /. x -> Infinity, 
      C[1] > 0]==1, C[1]]

{{C[1] -> 1/4}}

y[x_] := 4*Log[Cosh[x/4]];

And@@Join[eqns,{y'[0]==0,y'[Infinity]==1}]

True


Bob Hanlon
Chantilly, VA  USA


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