MHB Joko123's question at Yahoo Answers (Sketching a cone)

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The discussion revolves around demonstrating that the given parametric equations represent the curved surface of a circular cone with height h and base radius r. The equations show that as u varies from 0 to r and v from 0 to 2π, they describe circles in horizontal planes at varying heights, confirming the cone's structure. The transformation leads to the conical equation z² = (h²/r²)(x² + y²), which characterizes an unbounded conical surface. By analyzing the constraints, the sketch of the cone can be accurately created, illustrating the circular base and apex. The solution effectively clarifies how to visualize and understand the geometry of the cone.
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Here is the question:

Consider the Surface: r = ucos(v)i + usin(v)j + (h*u/r)k 0<u<r and 0<v<2*pi

(should be less then equal to for constraints)

Show that this represents the curved surface of a circular cone height h and base radius r. Sketch this cone?I have tried multiple times to solve this and can't seem to grasp a solution. PLEASE HELP

Here is a link to the question:

Please Help, How to Sketch a circular cone, height h and base r? - Yahoo!7 Answers

I have posted a link there to this topic so the OP can find my response.
 
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Hello Joko123,

Consider the surface: $$S:\left \{ \begin{matrix}x=u\cos v\\y=u\sin v\\z=\dfrac{h}{r}u\end{matrix}\right. \qquad (u\in\mathbb{R},\;v\in\mathbb{R})$$ We have $x^2+y^2=u^2\cos^2v+u^2\sin^2v=u^2(\cos^2v+\sin^2v)=u^2$ and $u=rz/h$, so: $$S:z^2=\frac{h^2}{r^2}(x^2+y^2)$$ and we know that a equation of this form is the equation of an unbounded conical surface. Now, consider the contraints $0\leq u\leq r,\;0\leq v\leq 2\pi$. For $u\in [0,r]$ we have $$\left \{ \begin{matrix}x=u\cos v\\y=u\sin v\\z=\dfrac{h}{r}u\end{matrix}\right. \qquad (v\in [0,2\pi])$$ that is, a circle on the plane $z=hu/r$.

If $u=0$, we get the point $(0,0,0)$ (circle with radius $0$).

If $u=r$, we get $$\left \{ \begin{matrix}x=r\cos v\\y=r\sin v\\z=h\end{matrix}\right. \qquad (v\in [0,2\pi])$$ that is, a circle on the plane $z=h$ with center at $(0,0,h)$ and radius $r$. Now, is easy to sketch the cone (choose only the part above):

 

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