# Problem solving separable diff EQ w/ I.Condition. , they say its wrong :x

1. Jan 14, 2006

### mr_coffee

Hello everyone i did this one surley thing it would work out and yet another failure.
Solve the separable differential equation
11 x - 8 y*sqrt{x^2 + 1}*{dy}/{dx} = 0.
Subject to the initial condition: y(0) = 6.
y = ?

I'm pretty sure where I messed up is when i tried to solve for y, i htink i screwed up there but i'm not sure, anyone know? Thanks! its #11. Ignore that top stuff please!

Here is my work:
http://img57.imageshack.us/img57/5705/lastscan8cm.jpg [Broken]

Last edited by a moderator: May 2, 2017
2. Jan 14, 2006

### benorin

Particular solution

You problem lies in where you placed your constant of integration, namely:

$$11\int \frac{x}{\sqrt{x^2+1}}dx = \frac{11}{2}\int \frac{1}{\sqrt{u}}du = \frac{11}{2}\int u^{-\frac{1}{2}}du = 11u^{\frac{1}{2}}+C=11\sqrt{x^2+1}+C$$

C is outside the square root, from there

$$4y^2=11\sqrt{x^2+1}+C$$

$$y^2=\frac{11}{4}\sqrt{x^2+1}+\frac{C}{4}$$

or,

$$y=\pm\sqrt{\frac{11}{4}\sqrt{x^2+1}+\frac{C}{4}}$$

to solve for C, use the simplest form of the solved DE, that is use

$$4y^2=11\sqrt{x^2+1}+C$$

for x=0 and y(0)=6, this gives

$$4(6)^2=11\sqrt{0^2+1}+C$$

which simplifies to

$$144=11+C$$

and hence C=133, plug this into

$$y=\pm\sqrt{\frac{11}{4}\sqrt{x^2+1}+\frac{C}{4}}$$

to get

$$y(x)=\pm\sqrt{\frac{11}{4}\sqrt{x^2+1}+\frac{133}{4}}=\pm\frac{1}{2}\sqrt{11\sqrt{x^2+1}+133}$$

Last edited: Jan 14, 2006
3. Jan 15, 2006

### leright

yup, that constant shouldn't be under the radical! :tongue:

4. Jan 15, 2006

### mr_coffee

Ahhh, that worked perfectly Benorin, thanks a ton (again)! :) The step by step explanation is great!