Euler's Equations for Extremas of J: y=C*e^x

[lylos]

Homework Statement

For the functional J(y(x))=\int^{x1}_{x2}F(x,y,y')dx, write out the curve y=y(x) for finding the extremas of J where F(x,y,y')=y'^2+y^2.

Homework Equations

Euler's Equations:
\frac{\partial f}{\partial y} - \frac{d}{dx}\frac{\partial f}{\partial y'}=0
\frac{\partial f}{\partial x} - \frac{d}{dx}(f-y' \frac{\partial f}{\partial y'})=0

The Attempt at a Solution

Using \frac{\partial f}{\partial y} - \frac{d}{dx}\frac{\partial f}{\partial y'}=0,
\frac{\partial f}{\partial y}=2y
2y=\frac{d}{dx}\frac{\partial f}{\partial y'}
2y=\frac{d}{dx}2y'
y=\frac{d^2y}{dx^2}
y=C*e^x Where C is a constant.

Is this correct? Using the 2nd equation, I get an ugly answer that involves Sinh.

 

[zaglamir]

Yo,

The general solution will be y=Ce^(lambda)x.

So you do y''-y=0

Which will look like: c(lambda)^2*e^x-c*e^x=0.

You can find lambda to be + or - 1. So your general solution will be C1*e^x+C2*e^-x=y.



Also, on the rest of our Homework III and IV, you can assume that it is a constant even if it has an x in it, Sophie said so.