# Homework Help: Equation of surface

1. Jul 14, 2011

### athrun200

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

It seems very obvious that after rotation, it becomes a circle, so how to write it out mathematically?

That I don't know the general equation of a surface.
Is $y^2+x^2=f^2(x)$ the answer?

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2. Jul 14, 2011

### Jaynte

Yes, this is the general equation for a circle

$$f(x)²=x²+y²=r²$$

As seen in the figure, f(x)=radius

And from polar coordinates you can get the equation:

$$x=r*cos(\Theta)$$
$$y=r*sin(\Theta)$$
$$x²+y²=r²(cos²(\Theta)+sin²(\Theta))=r²$$