- #1
mishima
- 565
- 35
This is a calculus of variations problem from Boas chapter 9. I seem to be misunderstanding something with differentiation. Given
$$F=(1+yy')^2$$
then
$$\frac {\partial F} {\partial y'}=2(1+yy')y$$
and
$$\frac {\partial F} {\partial y}=2(1+yy')y' .$$
Now this one I am not so confident on...
$$\frac {d}{dx} \frac {\partial F} {\partial y'}=2y'+(4yy'^2+2y^2y'')$$
because after Euler's equation,
$$\frac {d}{dx} \frac {\partial F} {\partial y'} - \frac {\partial F} {\partial y}=0$$
I get this strange differential equation
$$2yy'+2y^2y''=0.$$
Am I making a mistake with the derivatives or just not realizing the correct technique for the final differential equation?
$$F=(1+yy')^2$$
then
$$\frac {\partial F} {\partial y'}=2(1+yy')y$$
and
$$\frac {\partial F} {\partial y}=2(1+yy')y' .$$
Now this one I am not so confident on...
$$\frac {d}{dx} \frac {\partial F} {\partial y'}=2y'+(4yy'^2+2y^2y'')$$
because after Euler's equation,
$$\frac {d}{dx} \frac {\partial F} {\partial y'} - \frac {\partial F} {\partial y}=0$$
I get this strange differential equation
$$2yy'+2y^2y''=0.$$
Am I making a mistake with the derivatives or just not realizing the correct technique for the final differential equation?