Small Confusion with Partial Derivative

jegues
Messages
1,085
Reaction score
3

Homework Statement



Let u(x,y) = f(x^3 + y^2) +g(x^3 + y^2) such that f and g are differentiable functions. Show that,

2y\frac{\partial u}{\partial x} - 3x^{2} \frac{\partial u}{\partial y} = 0

Homework Equations





The Attempt at a Solution



The part of confused about is how to break down my partial derivatives.

The first thing I'm going to do is,

\text{Let } p=x^3 + y^2

then,

u = f(p) + g(p)

Now how to I extract,

\frac{\partial u}{\partial x},\frac{\partial u}{\partial y}

from here?

Is it simply,

\frac{\partial u}{\partial x} = \frac{du}{dp} \frac{\partial p}{\partial x}

The part that bothers me is that the du on the top is not a \partial.

Is this correct?
 
Physics news on Phys.org
jegues said:

Homework Statement



Let u(x,y) = f(x^3 + y^2) +g(x^3 + y^2) such that f and g are differentiable functions. Show that,

2y\frac{\partial u}{\partial x} - 3x^{2} \frac{\partial u}{\partial y} = 0

Homework Equations





The Attempt at a Solution



The part of confused about is how to break down my partial derivatives.

The first thing I'm going to do is,

\text{Let } p=x^3 + y^2

then,

u = f(p) + g(p)

Now how to I extract,

\frac{\partial u}{\partial x},\frac{\partial u}{\partial y}

from here?

Is it simply,

\frac{\partial u}{\partial x} = \frac{du}{dp} \frac{\partial p}{\partial x}

The part that bothers me is that the du on the top is not a \partial.

Is this correct?

I do believe it is. You're actually dealing with a composite function, which is why that du isn't partial.
 
Char. Limit said:
I do believe it is. You're actually dealing with a composite function, which is why that du isn't partial.

I think I'm confused because it says show that,

2y\frac{\partial u}{\partial x} - 3x^{2} \frac{\partial u}{\partial y} = 0

and in here I see,

\frac{\partial u}{\partial x} = \frac{du}{dp} \frac{\partial p}{\partial x}


Do the two d's cancel out and the two p's cancel out?
 
Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$ Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$ The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$ Plugging identity above into ##(1)## with ##u...
Back
Top