Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

You are not logged in.

## #1 2006-05-12 21:45:16

renjer
Member
Registered: 2006-04-29
Posts: 50

### Proving an Equation when the function is unknown

Now here's a problem I have, it's often that I know the function of f but in this particular question, I do not know what f is at all. Is it okay if I let f be a function of my choice and prove it from there?

Offline

## #2 2006-05-12 22:01:57

ganesh
Registered: 2005-06-28
Posts: 24,254

### Re: Proving an Equation when the function is unknown

renjer,
It is given w=f(x,y), x=rcosθ and y=rsinθ
and you are asked to show

You are right, w is given as a function of x and y, but the function is not defined

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Offline

## #3 2006-05-12 23:25:50

George,Y
Member
Registered: 2006-03-12
Posts: 1,316

### Re: Proving an Equation when the function is unknown

IT IS A GOOD QUESTION!!!

It's a question about coords, combination of derivatives, and vector calculus altogether.

It's about expressing the original combination of derivatives in another co-ords, usually orthogonal curvy ones. For example, polar co-ords are curvy, and dr and dθ at any given point are orthogonal to each other.

It's usually explained in a vector calculus book or a calculus book for physicians. Untill now, I haven't got a satisfactory explaination for this kind of transfer in any books.

However, I do find a particular solution for your coords and your case.

Define

After substition,

According to Chain Rule

Thus you can compute the right of your equation and use cos²θ+sin²θ=1 to prove it equates the left.

Last edited by George,Y (2006-05-12 23:39:22)

X'(y-Xβ)=0

Offline

## #4 2006-05-13 01:33:32

renjer
Member
Registered: 2006-04-29
Posts: 50

### Re: Proving an Equation when the function is unknown

I remember the chain rule telling me this:

So I'm quite unsure about your method from the chain rule onwards. Can you please explain further?

Offline

## #5 2006-05-13 01:59:16

ganesh
Registered: 2005-06-28
Posts: 24,254

### Re: Proving an Equation when the function is unknown

renjer,
In this link, at the bottom of the page, you would find the chain rule for partial derivatives, which I think, George has referred.

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Offline