1025
Concepts and Vocabulary
Multiple Choice A smooth surface has a [(a) normal plane, (b) tangent plane, (c) coordinate plane] at each point.
Multiple Choice Suppose \(x=x(u,v) ,\) \(y=y(u,v) ,\) \(z=z(u,v) \) are functions defined on a region \(R\) in the \(uv\)-plane. The set of points defined by \(( x,y,z) =( x(u,v) ,y(u,v) ,z(u,v) ) \) is called a(n) [(a) real surface, (b) uniform surface, (c) parametric surface.
Multiple Choice The parametric equations \(x=5\cos (u)\sin (v),\) \(y=5\sin (u)\sin (v),\) \(z=5\cos (v)\) define a [(a) plane, (b) circle, (c) sphere, (d) cylinder, (e) paraboloid].
Multiple Choice The parametrization \(\mathbf{r}(u,v)=( 1-u+2v) \mathbf{i}+5u\,\mathbf{j}+\left( 2u+3v-7\right) \mathbf{k}\) parametrizes a [(a) plane, (b) sphere, (c)line, (d) cylinder, (e) hyperboloid].
Skill Building
In Problems 5–8:
\(x(u,v) =u-5v, y(u,v) =2u, z(u,v) =-u+v+1\)
\(x(u,v) =u, y(u,v) =v\mathbf{,} z(u,v) =9-u^{2}-v^{2}\)
\(x(u,v) =u\cos v, y(u,v) =u\sin v, z(u,v) =u; 0\leq u\leq 2,\) \(0\leq v\leq \pi \)
\(x(u,v) =\cos u\sin v\,\mathbf{,} y(u,v) =\sin u\sin v, z(u,v) =\cos v; \\ 0\leq u\leq 2\pi , 0\leq v\leq \dfrac{\pi }{2}\)
In Problems 9–12, find a parametrization for each surface.
The part of the plane \(z=4-x-2y\) that lies in the first octant
The part of the surface \(z=e^{-x^{2}+y^{2}}\) that lies inside the cylinder \(x^{2}+y^{2}=4\)
The part of the surface \(z=\sin (x^{2}y)\) that lies above the region bounded by the graphs of \(y=x+2\) and \(y=x^{2}\)
The part of the surface \(y+e^{xz}=5\) that lies inside the cylinder \(x^{2}+z^{2}=1\)
In Problems 13–16:
\(\mathbf{r}(u,v) = ( 3u+2v)\mathbf{i}+5u^{3}\mathbf{j}+v^{2}\mathbf{k}\) at \((7,5,4)\)
\(\mathbf{r}(u,v) = ( 3u-v) \mathbf{i} + ( 2-u-v) \mathbf{j}+ ( 1+3v) \mathbf{k}\) at \((11,1,-5) \)
\(\mathbf{r}(u,v) =u\,\mathbf{i}+u\cos v\,\mathbf{j}+u\sin v\,\mathbf{k}\) at \( \left( 5,\dfrac{5\sqrt{2}}{2},\dfrac{5\sqrt{2}}{2 }\right) \)
\(\mathbf{r}(u,v) = ( 3\cos u\sin v+1) \, \mathbf{i}+ ( 2\sin u\sin v-1) \,\mathbf{j}+\cos v\,\mathbf{k}\) at \(\left( 1,0,\dfrac{\sqrt{3}}{2}\right) \)
In Problems 17–22, find the surface area of each surface \(S\).
\(S\) is parameterized by \(\mathbf{r}(u,v)=u\cos v \mathbf{i}+u^{3}\mathbf{j}+u\sin v\,\mathbf{k}\), \(0\leq u\leq 1,\) \(-\pi \leq v\leq \pi \).
\(S\) is parameterized by \(\mathbf{r}(u,v)=(3u^{2}+v)\mathbf{i}+u^{2}\mathbf{j}+(v-u^{2})\mathbf{k,} 0\leq u\leq 2, -1\leq v\leq 1\).
\(S\) is the part of the plane \(2x-y+4z=3\) that lies inside the cylinder \((x-2)^{2}+y^{2}=4\).
\(S\) is the part of the paraboloid \(z=4-x^{2}-y^{2}\) that lies above the \(xy\)-plane.
\(S\) is the part of the sphere \(x^{2}+y^{2}+z^{2}=16\) that lies above the plane \(z=2.\)
\(S\) is the frustum of the cone \(z=3\sqrt{x^{2}+y^{2}}\) that lies between \(z=3\) and \(z=6\).
In Problems 23–28, match each parametrization with its parametric surface.
\(\mathbf{r}\ (u,v) =u\cos v\,\mathbf{i}+u\sin v\, \mathbf{j} -u\,\mathbf{k,} 0\leq u\leq 4, 0\leq v \leq 2\pi \)
\(\mathbf{r}\ (u,v) =u^{3}\,\mathbf{i}+u\sin v\, \mathbf{j}+u\cos v\,\mathbf{k,} 0\leq u\leq 4, 0\leq v \leq 2\pi \)
\(\mathbf{r}\ (u,v) =u\sin v\,\mathbf{i}+u\cos v\, \mathbf{j}+u\,\mathbf{k,} 0\leq u\leq 4, 0\leq v \leq 2\pi \)
\(\mathbf{r}\ (u,v) =u\,\mathbf{i}+4\sin v\, \mathbf{j}+4\cos v\,\mathbf{k,} 0\leq u\leq 4, 0\leq v \leq 2\pi \)
\(\mathbf{r}\ (u,v) =u\cos v\,\mathbf{i}+u\sin v\, \mathbf{j}-u^{3}\,\mathbf{k,} 0\leq u\leq 4, 0\leq v \leq 2\pi \)
\(\mathbf{r}\ (u,v) =v\,\mathbf{i}+v^{3}\,\mathbf{j}+u\,\mathbf{k,} 0\leq u\leq 4, -4\leq v \leq 4\)
1026
Applications and Extensions
Parametrize the part of the cylinder \(x^{2}+y^{2}=16\) that lies above the \(xy\)-plane and below \(z=3\).
Parametrize the sphere \(x^{2}+y^{2}+z^{2}=25\).
Part of the paraboloid \(z=9-x^{2}-y^{2}\) lies inside the cylinder \((x-1)^{2}+y^{2}=1\).
Parametrize the lumpy sphere \(x^{2}+y^{2}+z^{2}=3 \sqrt{x^{2}+y^{2}+z^{2}}+z\).
(Hint: Use spherical coordinates.)
In Problems 33 and 34, parametrize each surface:
The part of the sphere \(x^{2}+y^{2}+z^{2}=4\) lying in the first octant
The plane \(x-\sqrt{3}y=0,\) where \(x\geq 0\), \(y\geq 0\)
Tangent Plane to a Torus
Surface Area Find the surface area \(S\) of the torus parametrized by \(\mathbf{r}(u,v)=(2+\cos v)\cos u\mathbf{i}+(2+\cos v)\sin u \mathbf{j}+\sin v\mathbf{k}\), \(0\leq u\leq 2\pi ,\) \(\ 0\leq v\leq 2\pi \).
Surface Area Find the surface area \(S\) of the helicoid parametrized by \(\mathbf{r}(u,v)=u\cos (2v)\mathbf{i}+u\sin (2v)\mathbf{j}+v \mathbf{k}\), \(0\leq u\leq 1,\) \(0\leq v\leq 2\pi \).
Surface Area Find the surface area \(S\) of the Möbius strip parametrized by \(\mathbf{r}(u,v)=\left( 2\cos {u}+ v\cos\dfrac{u}{2}\right) \mathbf{i}+\left( 2\sin u+v \cos \dfrac{u}{2}\right) \mathbf{j} -v\sin \dfrac{u}{2}\,\mathbf{k}\), \(0\leq u\leq 2\pi ,\) \(-0.5 \leq v\leq 0.5\).
Ellipsoid
Helicoid
Dini's Surface
Challenge Problems
Suppose \(\Delta ABC\) is a triangle with vertices \( A=(a_{1},a_{2},a_{3})\), \(B=(b_{1},b_{2},b_{3})\), and \(C=(c_{1},c_{2},c_{3})\) . Parametrize \(\Delta ABC\).
Torus A torus is formed by rotating a circle about another circle.
Surface Area Find the surface area of \(S\), where \(S\) is the part of the cylinder \((x-1)^{2} + y^{2} = 1\) outside the sphere \(x^{2}+y^{2} + z^{2} = 4\), above the \(xy\)-plane, and below \(z=1\).
An Archimedean Ratio Suppose \(C\) is a right circular cone, \(S\) is an upper hemisphere, and \(L\) is a right circular cylinder. Suppose that all three surfaces have radii \(r\) and the heights \(h\) of the cone and the cylinder are \(r\). Archimedes was able to show that the ratio of the surface areas among these surfaces is \(\sqrt{2}:2:2\). Show this by parametrizing each of these surfaces and deriving the formulas for their surface areas.
Hyperboloid of One Sheet
Hyperboloid of Two Sheets