- #1
x-is-y
- 5
- 0
If [tex]z = f(x,y)[/tex] and [tex]x = r \cos{v}[/tex], [tex] y = r\sin{v}[/tex] the object is to show that [tex] d = \partial [/tex] since it's easier to do on computer
Show that:
[tex] \frac{d^2 z}{dr^2} + \frac{1}{r} \frac{dz}{dr} + \frac{1}{r^2} \frac{d^2 z}{dv^2} = \frac{d^2 z}{dx^2} + \frac{d^2 z}{dy^2} [/tex]
It's from Adams calculus, will show where I get lost.
We have from the chain rule
[tex] \frac{dz}{dr} = \frac{dz}{dx} \frac{dx}{dr} + \frac{dz}{dy} \frac{dy}{dr} [/tex]
But [tex] \frac{dx}{dr} = \cos{v} [/tex] and [tex] \frac{dy}{dr} = \sin{v} [/tex], so gives:
[tex] \frac{dz}{dr} = \frac{dz}{dx} \cos{v} + \frac{dz}{dy} \sin{v} [/tex]
Now we need [tex] \frac{d^2 z}{dr^2} [/tex], where I end up in trouble. We have
[tex] \frac{d^2 z}{dr^2} = \frac{d}{dr} \cos{v} \frac{dz}{dx} + \frac{d}{dr} \sin{v} \frac{dz}{dy} [/tex] which can be written as
[tex] \frac{d^2 z}{dr^2} = \cos{v} \frac{d}{dr} \frac{dz}{dx} + \sin{v} \frac{d}{dr} \frac{dz}{dy} [/tex]
Now my books says that
[tex] \frac{d}{dr} \frac{dz}{dx} = \cos{v} \frac{d^2 z}{dx^2} + \sin{v} \frac{d^2 z}{dy dx} [/tex] if I understand correct. But I don't see how you get to this ...
Show that:
[tex] \frac{d^2 z}{dr^2} + \frac{1}{r} \frac{dz}{dr} + \frac{1}{r^2} \frac{d^2 z}{dv^2} = \frac{d^2 z}{dx^2} + \frac{d^2 z}{dy^2} [/tex]
It's from Adams calculus, will show where I get lost.
We have from the chain rule
[tex] \frac{dz}{dr} = \frac{dz}{dx} \frac{dx}{dr} + \frac{dz}{dy} \frac{dy}{dr} [/tex]
But [tex] \frac{dx}{dr} = \cos{v} [/tex] and [tex] \frac{dy}{dr} = \sin{v} [/tex], so gives:
[tex] \frac{dz}{dr} = \frac{dz}{dx} \cos{v} + \frac{dz}{dy} \sin{v} [/tex]
Now we need [tex] \frac{d^2 z}{dr^2} [/tex], where I end up in trouble. We have
[tex] \frac{d^2 z}{dr^2} = \frac{d}{dr} \cos{v} \frac{dz}{dx} + \frac{d}{dr} \sin{v} \frac{dz}{dy} [/tex] which can be written as
[tex] \frac{d^2 z}{dr^2} = \cos{v} \frac{d}{dr} \frac{dz}{dx} + \sin{v} \frac{d}{dr} \frac{dz}{dy} [/tex]
Now my books says that
[tex] \frac{d}{dr} \frac{dz}{dx} = \cos{v} \frac{d^2 z}{dx^2} + \sin{v} \frac{d^2 z}{dy dx} [/tex] if I understand correct. But I don't see how you get to this ...