# Blog

Problem 1.  (a) First, you likely need to draw a picture of the two bounding surfaces (the part of the sphere and the cone).  From this picture one can see that the sphere is “on top” and the cone is “on the bottom.”  This lets us describe the solid region using the variable $z$ first.

The solid region can be described by:

$\sqrt{x^2+y^2} \leq z \leq \sqrt{4-x^2-y^2}$

$\displaystyle -\sqrt{2-x^2} \leq y \leq \sqrt{2-x^2}$

$\displaystyle -\sqrt{2} \leq x \leq \sqrt{2}$

The $x$ and $y$ descriptions / inequalities are obtained from our picture and from solving the equations $\sqrt{x^2+y^2} = \sqrt{4-x^2-y^2}$ to find that $x^2+y^2 = 2$.

The volume is then given by

$\displaystyle \text{Vol}(S) = \iiint_S \, dV = \int_{-\sqrt{2}}^{\sqrt{2}} \, \int_{-\sqrt{2-x^2}}^{\sqrt{2-x^2}}\,\int_{\sqrt{x^2+y^2}}^{\sqrt{4-x^2-y^2}} \, dz\,dy\,dx.$

(b) To actually evaluate this volume, one should change variables.  Switching either to cylindrical or to spherical coordinates can make this problem do-able.  Here is the cylindrical coordinate way.

(Cylindrical Coordinates).  The region $S$ can be described by the inequalities

$\displaystyle r \leq z \leq \sqrt{4-r^2}$

$\displaystyle 0 \leq r \leq \sqrt{2}$

$\displaystyle 0 \leq \theta \leq 2\pi$.

We then apply our change-of-variables formula to compute

$\displaystyle \text{Vol}(S) = \iiint_S \, dV = \int_0^{2\pi} \, \int_0^{\sqrt{2}} \, \int_r^{\sqrt{4-r^2}} \, r\, dz\, dr\,d\theta$

Notice that if we compute the $\theta$ integration first we find

$\displaystyle \text{Vol}(S) = 2\pi \, \int_0^{\sqrt{2}}\, \int_r^{\sqrt{4-r^2}} \, r\, dz\, dr = 2\pi \int_0^{\sqrt{2}} r\,\left(\sqrt{4-r^2} - r\right)\,dr$

and this integral equals

$= 2\pi\int_0^{\sqrt{2}} r\sqrt{4-r^2}\,dr - 2\pi\int_0^{\sqrt{2}} r^2\, dr$.

The first of these integrals can be evaluated by applying a $u$-substitution of $u = 4-r^2$.

(Spherical Coordinates).  This integral is probably easier to do in spherical coordinates.  Indeed, the region of integration is easily described by the following inequalities:

$\displaystyle 0 \leq \rho \leq 2$

$\displaystyle 0 \leq \theta \leq 2\pi$

$\displaystyle 0 \leq \varphi \leq \pi/4$

The change-of-variables formula gives us

$\displaystyle \text{Vol}(S) = \iiint_S \! dV = \int_0^{\pi/4} \, \int_0^{2\pi} \, \int_0^2 \, \rho^2\sin\varphi\,d\rho\,d\theta\,d\varphi$

which can be computed relatively easily.

Problem 2.  (a) $F(1,0,0) = (a, 0,0), F(0,1,0) = (0, b, 0)$ and $F(0,0,1) = (0, 0, b)$.  The unit sphere is sent to the ellipsoid whose equation is

$\displaystyle \frac{x^2}{a^2} + \frac{y^2}{b^2} = \frac{z^2}{c^2} = 1$.

In particular, $F$ stretches the $x$-axis by a factor of $a > 0$, it stretches the $y$-axis by a factor of $b>0$, and it stretches the $z$-axis by a factor of $c > 0$.

(b) The derivative matrix of $F$ is easy to compute.  One finds

$\displaystyle DF = \left[ \begin{array}{ccc} a & 0 & 0 \\ 0 & b & 0 \\ o & o & c \end{array}\right]$.

The determinant of this matrix is $\det DF = abc$.

(c ) To compute the volume of $E$ we can change variables using the function $F$.  Based on our observations in part (a) we know that if $S$ is the unit sphere, then $F(S) = E$ and so we can convert a volume integral over $E$ as follows:

$\displaystyle \text{Vol}(E) = \iiint_E \, dz\,dy\,dx = \iiint_{F(S)} \, dz\,dy\,dx = \iiint_S \, \left|\det\,DF\right|\,du\,dv\,dw\, = abc\iiint_S \, du\, dv\, dw.$

This last triple integral computes the volume bounded by the unit sphere $S$, and we computed that to be $4\pi/3$.  Therefore, the volume bounded by the ellipsoid $E$ is given by

$\displaystyle \text{Vol} = \frac{4\pi}{3}\,a\cdot b\cdot c$.

Problem 3.  This was done in class.

Problem 4.  (a) The length of $\gamma$ is given by

$\displaystyle \text{L}(\gamma) = \int_{\gamma} \, ds = \int_0^{2\pi} \, \left|\vec{r}'(t)\right|\,dt = \int_0^{2\pi} \, \sqrt{\sin^2t+\cos^2t + 1} = \int_0^{2\pi} \, \sqrt{2}\, dt = 2\pi\sqrt{2}$.

(b) In this problem, the total mass of $\gamma$ is given by the line integral $\int_{\gamma} \, f\,ds$.  This can be evaluated directly:

$\displaystyle \int_{\gamma} \, f\, ds = \int_0^{2\pi} \, f(\vec{r}(t))\,\left|\vec{r}'(t)\right|\,dt = \sqrt{2}\int_0^{2\pi} \cos^2t - \sin^2t + t^2\, dt.$

This integral equals $\displaystyle \sqrt{2}\,\frac{8\pi^3}{3}$.

(c ) The work done by the given vector field (along $\gamma$) is given by

$\displaystyle \int_{\gamma} F\cdot d\vec{s} = \int_0^{2\pi} \, F(\vec{r}(t))\cdot\vec{r}'(t)\,dt$.

One can compute the dot product in the integrand as follows:

$\displaystyle F(\vec{r}(t))\cdot\vec{r}'(t) = (-\sin t, \cos t, t)\cdot (-\sin t, \cos t, 1) = 1+t$.

Hence, the total work done is

$\displaystyle \int_0^{2\pi} \, 1+t\, dt = \left[t + \frac{t^2}{2}\right]_0^{2\pi} = 2\pi + 2\pi^2.$

Problem 5.  This follows by applying the definition of gradient and curl.  In particular, given such a function $f(x,y,z)$ we have that $\nabla f = \left(f_x, f_y, f_z\right)$.  The curl of the gradient is then given by

$\displaystyle \nabla \times \nabla f = \left|\begin{array}{ccc} \vec{i} & \vec{j} & \vec{k} \\ \partial_x & \partial_y & \partial_z \\ f_x & f_y & f_z \end{array}\right|.$

The resulting vector field is given by

$\displaystyle = \left(f_{zy} - f_{yz}\right)\vec{i} - \left(f_{xz} - f_{zx}\right)\vec{j} + \left(f_{xy} - f_{yx}\right)\vec{k} = (0, 0, 0)$

since mixed partials are equal.

Problem 6. (a) This line integral can be computed directly, or one can notice that $F = \nabla f$ where $f(x,y) = e^{xy}$.  By the Gradient Theorem, we then have

$\displaystyle \int_{\gamma} F\cdot d\vec{s} = f(4,0) - f(4,0) = 0$.

(b) We can apply the same theorem, albeit now to the slightly different curve $\alpha$, whose start- and end-points are $(4,0)$ and $(-4,0)$, respectively.  We find

$\displaystyle \int_{\alpha} F\cdot d\vec{s} = f(-4, 0) - f(4,0) = e^0 - e^0 = 0$.

Problem 7.  Here are the statements.

Gradient Theorem.  If $F = \nabla f$, then

$\displaystyle \int_{\gamma} F\cdot d\vec{s} = f(\text{end point}) - f(\text{start point})$

Green’s Theorem.  If $D$ is a region in the plane with a correctly oriented boundary, $\partial D$, then

$\displaystyle \iint_D \, \left(\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y}\right)\, dA = \int_{\partial D}\, \left(P, Q\right)\cdot d\vec{s}$

note: if $\partial D$ is parameterized by a function $\vec{r}(t) = (x(t), y(t))$, then some textbooks will notate $d\vec{s} = (x'(t), y'(t))dt$ and rewrite $x'(t) dt = dx$ and $y'(t) dt = dy$.  Given this notation, the line integral can also be written as

$\displaystyle \int_{\partial D} Pdx + Qdy$.

Stokes’ Theorem.  If $S$ is an orientable surface with boundary curve(s) $\partial S$, then

$\displaystyle \iint_S \, \left(\nabla \times F\right) \cdot d\vec{S} = \int_{\partial S} \, F \cdot d\vec{s}$.

Here, the line integral is over the oriented curve(s) $\partial S$ whose directions are compatible with the choice of unit normal for the surface $S$.

Divergence Theorem.  If $W$ is a solid region in space with boundary surface $\partial W$, then

$\displaystyle \iiint_W \, \text{div} F\, dV = \iint_{\partial W} \, F \cdot d\vec{S}$

where the surface integral is over $\partial W$ with an outward-pointing normal.

Problem 8.  The area of $D$ is given by

$\displaystyle \text{Area}(D) = \iint_D \, dA$.

If we can find a vector field $F(x,y) = (P(x,y), Q(x,y))$ where $Q_x - P_y = 1$ then we can apply Green’s Theorem to find

$\displaystyle \text{Area}(D) = \iint_D \, dA = \int_{\partial D} \, F\cdot d\vec{s}.$

There are many, many such vector fields $F$ to choose from.  One choice is to use $P(x,y) = 0$ and $Q(x,y) = 1$.  We can then compute the line integral by parameterizing the ellipse-region using $\vec{r}(t) = (2\cos t, 5\sin t)$ for $0 \leq t \leq 2\pi$.  (Note: this parameterization gives the boundary curve the correct direction.)  We then have

$\displaystyle \text{Area}(D) = \int_0^{2\pi} \, F(\vec{r}(t))\cdot\vec{r}'(t)\,dt = \int_0^{2\pi} 0\cdot (-2\sin t) + (5\cos t)\cdot(5\cos t)\,dt$

This integral equals

$\displaystyle = \int_0^{2\pi}\, 25\cos^2(t)\, dt = 25\pi$

which can be computed by using a double-angle identity.

Problem 9.  In general, the surface area for a surface $S$ is given by

$\displaystyle \text{Surface Area}(S) = \iint_S \, dS = \iint_D \, \left|\vec{r}_s\times\vec{r}_t\right|\,dA$

where $\vec{r}(s,t) : D \to \mathbb{R}^3$ parameterizes $S$.  We can parameterize a graph-surface, $S = \left\{(x,y,f(x,y)) : (x,y) \in D\right\}$ by simply using

$\displaystyle \vec{r}(s,t) = (s, t, f(s, t))$.

In other words, we are using $x = s$ and $y = t$.  As worked out in our textbook in 11.6 (on page 231), this becomes

$\displaystyle \iint_D \, \sqrt{f_s^2 + f_t^2 + 1} \, ds\, dt$

or, if we change notation as is done in the book and use $x = s$ and $y = t$, then we have

$\displaystyle \text{Surface Area}(S) = \iint_D \, \sqrt{f_x^2 + f_y^2 + 1}\, dx\, dy.$

For our function we have that $f(x,y) = x^2+y^2$ and so $f_x = 2x$ and $f_y = 2y$.  The surface area is then given by

$\displaystyle \text{Surface Area}(S) = \iint_D \, \sqrt{4x^2+4y^2 + 1}\, dx\, dy = \int_{-2}^2 \, \int_{-\sqrt{4-y^2}}^{\sqrt{4-y^2}}\, \sqrt{4x^2+4y^2+1}\,dx\,dy.$

Problem 10.  (a) The symbols translate as follows:

$ds = \left|\vec{r}'(t)\right|\, dt, \,\, d\vec{s} = \vec{r}'(t)\, dt$

$dS = \left|\vec{r}_s \times \vec{r}_t\right|\,ds\,dt, \, \, d\vec{S} = \left(\vec{r}_s\times\vec{r}_t\right)\,ds,dt$.

(b) The first two are used for line integrals, and the last two are used for surface integrals.

(c ) Again, the surface area of a parameterized $S$ is given by

$\displaystyle \text{Surface Area}(S) = \iint_S \, dS = \iint_D \, \left|\vec{r}_s\times\vec{r}_t\right|\,ds\,dt$.

A unit normal is given by

$\displaystyle \vec{n} = \frac{\vec{r}_s\times\vec{r}_t}{\left|\vec{r}_s\times\vec{r}_t\right|}.$

Problem 11.  To compute the flux of $F$ out of $S$, we need an outward pointing (unit) normal for $S$.  This requires us to first parameterize the unit sphere $S$, which we can do as follows:

$\displaystyle \vec{r}(s, t) = \left(\sin t\, \cos s, \, \sin t\,\sin s, \, \cos t\right)$

where $s \in [0, 2\pi]$ and $t \in [0, \pi]$.  This parameterization comes from spherical coordinates, thinking of $s = \theta$ and $t = \varphi$.  When we are on the unit sphere, the only restriction (on spherical coordinates) is the equation $\rho = 1$, leaving $\theta$ and $\varphi$ free to roam within their respective intervals.  If we then recall our conversion formulas relating $x, y, z$ to $\rho, \theta, \varphi$ and set $\rho = 1$, we obtain the above expression.

A unit normal for $S$ can then be computed by evaluating

$\displaystyle \vec{n} = \pm \frac{\vec{r}_s\times\vec{r}_t}{\left|\vec{r}_s\times\vec{r}_t\right|}$.

The computation above requires some time to do, but when it is all said and done (and terms are cancelled), we find

$\displaystyle \vec{n} = \pm (\sin t\,\cos s, \, \sin t\,\sin s, \, \cos t)$.

An outward normal is obtained by using the plus sign in the equation above.  This gives us a normal, for example, that points up, out of the sphere, at the north pole $(0, 0, 1)$.  We also find

$\displaystyle dS = \left|\vec{r}_s\times\vec{r}_t\right| = \sin t\,ds\,dt$.

We can then evaluate this flux as

$\displaystyle \iint_S \, F\cdot d\vec{S} = \iint_S \left(F\cdot\vec{n}\right)\,dS = \int_0^{\pi}\,\int_0^{2\pi}\, \sin t\, ds\, dt =4\pi$

The integrand $F\cdot\vec{n}$ cancels beautifully; in fact, it equals the constant $1$.

(Slightly different approach) One can also approach this problem by simply writing

$\displaystyle d\vec{S} = (\vec{r}_s\times\vec{r}_t)\,ds\,dt = \sin t\,(\sin t\,\cos s, \, \sin t\,\sin s, \, \cos t)\,ds\,dt$

and then writing out

$\displaystyle F(\vec{r}(s,t)) = (x(s, t), y(s, t), z(s, t)) = (\sin t \, \cos s, \, \sin t \, \sin s, \, \cos t)$.

One can use these expressions to explicitly compute the flux as

$\displaystyle \iint_S \, F\cdot d\vec{S} = \int_0^{\pi} \, \int_0^{2\pi} \, F(\vec{r}(s, t))\cdot\left(r_s\times r_t\right)\,ds\,dt$.

(Another approach)  One can also observe that, for the unit sphere, $\vec{n} = (x,y,z)$.  Note that this expression for the unit normal does not come from a parameterization, but instead comes from viewing the unit sphere as a level set

$\displaystyle x^2+y^2+z^2 = 1$.

The gradient of the level-set function $G(x,y,z) = x^2+y^2+z^2$ is perpendicular to the level set, and so this gradient can be used as a normal.  One finds $\nabla G = (2x, 2y, 2z)$.  To make this vector have unit length we divide by $2$ to get $(x, y, z)$ (which has unit length since the point lies on the unit sphere).

Observe that the outward normal for the sphere and the vector field, $F$, in this problem are the same!  We then have

$F\cdot n = x^2+y^2+z^2 = 1$

when $(x,y,z)$ lies on the sphere.  This implies that the flux is given by

$\displaystyle \iint_S \, F\cdot\vec{n}\,dS = \iint_S \, dS = \text{Surface Area}(S)$.

This method still leaves one with computing the surface area of the unit sphere, which is probably best done using a parameterization (although some might recall the formula).

(Crazy Cool Approach)  We computed the volume of the solid region bounded by the unit sphere in a previous class and found this volume to be $4\pi/3$.  Note that the given sphere, $S$, is the boundary of the filled-in, solid ball, $B = \{(x,y,z) : x^2+y^2+z^2 \leq 1\}$, and so we may use the Divergence Theorem to compute the flux:

$\displaystyle \iint_S \, F\cdot d\vec{S} = \iint_{\partial B} \, F \cdot d\vec{S} = \iiint_B \, \text{div}(F)\,dV = 3\iiint_B \, dV = 3\cdot \frac{4\pi}{3} = 4\pi$.

Problem 12.  (a) To compute this flux directly, we need to first parameterize the surface.  This can be done by using the function

$\displaystyle \vec{r}(s, t) = (\cos s, \sin s, t)$

where $s \in [0, 2\pi]$ and $-1 \leq t \leq 1$.  This parameterization is motivated by cylindrical coordinates (with $s = \theta, t = z$ and $r = 1$ since it is a radius 1 cylinder).

We then compute

$\displaystyle d\vec{S} = \left(\vec{r}_s\times\vec{r}_t\right)\,ds\,dt = (-\sin s, \cos s, 0) \times (0, 0, 1) \, ds\, dt = (\cos s, \sin s, 0)\, ds\, dt$.

The flux integral is then given by

$\displaystyle \iint_S \, F\cdot d\vec{S} = \int_{-1}^1 \, \int_0^{2\pi} \, \big{(}\,(-\sin s, \cos s, t)\cdot(\cos s, \sin s, 0)\big{)}\,ds\,dt = \int_{-1}^1\,\int_0^{2\pi}\,0\,ds\,dt$

and so equals zero.

Note: a student might try to do this problem by using, say, the divergence theorem.  After all, the divergence of this vector field is easy to compute, but this is not applicable since the cylinder $S$ is not the boundary of a solid region!  If it contained the “top lid” and “bottom lid,” then it would be.

A student might also try to do this problem by using, say, Stokes’ Theorem.  To do this, though, one would need to know that $\nabla \times V = F$ for some vector field $V$ — however, it is hard to find such a vector field $V$, and, more over, it does not exist since $\text{div}(F) \neq 0$.

(b) $S$ is not the boundary of a solid region.  A picture explains this.

(c ) The boundary of $S$ consists of two curves, $\gamma_1$ and $\gamma_2$.  Each curve is a circle, one contained in the $z = 1$ plane and one contained in the $z = -1$ plane.

The curve $\gamma_1$ in the $z = -1$ plane must be oriented in a counter-clockwise direction to be consistent with an outward normal for $S$.  The curve $\gamma_2$ in the $z = 1$ plane must be oriented in a clockwise direction, though!

(d) For this problem one can compute the curl $\nabla \times F$ directly and compute this flux integral directly.  This is a valid way to do this problem since the curl is not that hard to compute.

However, integrating $\nabla \times F$ begs us to use Stokes Theorem.  It says that

$\displaystyle \iint_S \left(\nabla \times F\right)\cdot d\vec{S} = \int_{\gamma_1} F\cdot d\vec{s} + \int_{\gamma_2} F\cdot d\vec{s}$.

One can parameterize $\gamma_1$ using the function $\vec{r}_1(t) = (\cos t, \sin t, -1)$ and one can parameterize $\gamma_2$ using the function $\vec{r}_2(t) = (\cos t, \sin t, 1)$.  Both parameterizations have the domain $t \in [0, 2\pi]$, but the function $\vec{r}_2(t)$ traverses $\gamma_2$ in the wrong direction, and so we must adjust our line integral with a negative sign.

Both line integrals equal $2\pi$, and so when we subtract we (of course) find that

$\displaystyle \iint_S \, F\cdot d\vec{S} = 0$.

Problem 13.  We can use the divergence theorem to kill this problem.  Observe that the divergence of $F$ is given by $\text{div}(F) = 3$ and so

$\displaystyle 30 = 3\cdot \text{Vol}(W) = 3\cdot \iiint_W \, dV = \iiint_W \, 3dV = \iiint_W \, \text{div}(F)\,dV$

and by the Divergence Theorem this last integral equals

$\displaystyle \iiint_W \, \text{div}(F)\,dV= \iint_{\partial W} \, F\cdot d\vec{S}$.

Therefore the flux out of $\partial W = S$ is $30.$

## Assignment 11

Problem 1.  (a) Set up a triple integral (using $dV = dz\,dy\,dx$) that represents the volume of the solid region $S \subset \mathbb{R}^3$ bounded by the sphere $x^2+y^2+z^2 = 4$ and the cone $z = \sqrt{x^2+y^2}$.

(b) Change variables for your integral in part (a) so that you can compute the volume of $S$.

Problem 2.  For this problem we will use a function $F:\mathbb{R}^3\to\mathbb{R}^3$ that depends on three positive, constant numbers $a, b, c \in \mathbb{R}$:

$\displaystyle F(u,v,w) = (au, bv, cw)$.

(a) What is $F(1,0,0)$?  What is $F(0, 1, 0)$ and what is $F(0,0,1)$?  Describe in words and/or pictures what $F$ does to the unit sphere (centered at the origin).

(b) Compute the derivative matrix of $F$ and compute the determinant of this derivative matrix.

(c)  Compute the volume of the region bounded by the ellipsoid $E \subset \mathbb{R}^3$ given by the equation

$\displaystyle \frac{x^2}{a^2} + \frac{y^2}{b^2} + \frac{z^2}{c^2} = 1.$

Problem 3.  Evaluate the (improper, one-dimensional) integral

$\displaystyle \int_{-\infty}^{\infty} \, e^{-x^2}\, dx.$

Problem 4.  Consider the helix curve $\gamma$ that is parameterized by the function $\vec{r}(t) = (\cos t, \sin t, t)$ where $t \in [0, 2\pi]$.

(a) Compute the length of the curve $\gamma$.

(b) If the function $f(x,y,z) = x^2-y^2+z^2$ measures the density at every point $(x,y,z)$ on $\gamma$, compute the total mass of $\gamma$.

(c) Compute the amount of work done by the vector field $F(x,y,z) = (-y, x, z/(x^2+y^2)\,)$ in moving a particle along $\gamma$.

Problem 5.  Let $f:\mathbb{R}^3\to\mathbb{R}$ be any differentiable function.  Prove / compute that

$\displaystyle \nabla \times \nabla f = \text{curl}\,\left(\nabla f\right) = \vec{0}.$

Problem 6.  (a) Compute the line integral

$\displaystyle \int_{\gamma} \, F\cdot d\vec{s}$

where $F(x,y) = (ye^{xy}, xe^{xy})$ and $\gamma$ is the ellipse (traversed in a counter-clockwise direction) given by

$\displaystyle \frac{x^2}{16} + \frac{y^2}{9} = 1$.

(b) Compute the line integral

$\displaystyle \int_{\alpha} \, F\cdot d\vec{s}$

where $F$ is the same vector field from part (a), but $\alpha$ is only the top half of the ellipse from part (a) (also given a counter-clockwise orientation).

Problem 7.  State the following integral theorems:

The Gradient Theorem, Greens’ Theorem, Stokes’ Theorem, the Divergence Theorem.

Problem 8.  Use Greens Theorem to compute the area of the region $D \subset \mathbb{R}^2$ given by

$\displaystyle D = \left\{ (x,y) : \frac{x^2}{4} + \frac{y^2}{25} \leq 1 \right\}.$

Problem 9.  Set up a double integral that represents the surface area of the graph for $f(x,y) = x^2+y^2$ over the domain $\left\{(x,y) : x^2+y^2 \leq 4\right\}$.

Problem 10.  (a) Write down the meaning of the following symbols:

$ds =$

$d\vec{s} =$

$dS =$

$d\vec{S} =$

(b) Which of the above symbols are used for line integrals?  Which are used for surface integrals?

(c) Given a surface $S \subset \mathbb{R}^3$, write down an integral expression that represents its surface area (assuming the surface has been parameterized by a function $\vec{r}(s, t) : D \to \mathbb{R}^3$).  In addition, write down an expression for a unit normal to $S$.

Problem 11.  Compute the flux of the vector field $F(x,y,z) = (x, y, z)$ out of the unit sphere $S = \left\{(x,y,z) : x^2+y^2+z^2 = 1\right\}$.

Problem 12.  (a) Compute the flux of the vector field $F(x,y,z) = (-y,x,z)$ out of the cylindrical surface

$\displaystyle S = \left\{(x,y,z) : x^2+y^2 = 1 \text{ and } -1 \leq z \leq 1\right\}$.

(b) Draw a picture of the surface $S$.  Is this surface the boundary of a solid region?  Why or why not?

(c) Describe / draw a picture of the boundary of $S$ or explain why it has no boundary.  If $S$ does have a boundary, how should it be oriented so as to be consistent with the outward normal $n$?

(d) Compute the flux

$\displaystyle \iint_S \, \left(\nabla \times F\right)\,\cdot\,d\vec{S}$.

Problem 13.  Suppose $W \subset \mathbb{R}^3$ is a solid region that has volume $\text{Vol}(W) = 10$.  If $S = \partial W$ is the boundary surface of $W$, compute the flux of $F$ out of the surface $S$ where

$F(x,y,z) = (x+y^2+z^5, \cos(xz)e^z + y, xy + \arctan(xy) + z)$

## Assignment 10

Read and take (careful) notes over Sections 5.1 and 5.2 from our textbook.  Please fill in some of the details skipped in class regarding Theorem 5.1 and Example 5.4.

Lastly, write a short (or long, if you prefer) response to this prompt:  “This Thanksgiving I am thankful for Alex Meadows because…”

As usual, I’d like to share your responses with Alex, so please compose this on a separate sheet of paper.  Also, if you would prefer to remain anonymous you may do so by omitting any and all identifying information.

## Assignment 10

Reading: 11.1, 11.2, 11.3, 11.4, and 11.7 (these are all of the sections on double and triple integrals).  Please note that in class we defined a double integral and a triple integral as

$\displaystyle \iint_R \! f\, dA = \text{ limit of (signed) 3-dimensional box volumes}$

$\displaystyle \iiint_B \! f\, dV = \text{ limit of (signed) 4-dimensional "box" volumes}$

there are other interpretations and applications of double and triple integrals.  As mentioned in our textbook, we can also interpret these integrals as telling us about average values of functions, as telling us about the total mass of an object, and even as telling us about the probability of a particular event taking place.  Please read sections 11.4 and 11.7 carefully for alternate interpretations of and uses for these integrals!  In particular, I strongly recommend reading through the summaries of these sections!

Problem 1.  Compute the volume of the solid region bounded by the graph of $z = f(x,y) = ye^x$ and the rectangle $R = [0, \ln 2] \times [-1, 1]$ in the $xy$-plane.

Problem 2.  Complete Activity 11.6 (parts a and b) from our textbook.

Problem 3.  Complete Activity 11.8 (parts a through e) from our textbook.

Problem 4.  Exercise 4(a) – 4(c) from page 207 in our textbook.

Problem 5.  Suppose $f:\mathbb{R}^2\to\mathbb{R}$ is a given function with average value $10$ over the set $D = \left\{ \left(x,y\right) : y \leq x \leq 1 \text{ and } 0 \leq y \leq 2 \right\}$.

(a) Draw a picture of the region $D$.

(b) Compute the area of $D$.

(c ) Compute the value of the double integral

$\displaystyle \iint_D \! f\, dA.$

Problem 6.  Evaluate the triple integral $\displaystyle \iiint _B \! f\, dV$ where $f(x,y,z) = xy + e^{z+y}$ and $B = [1, 2] \times [1, 3] \times [-2, \pi]$.

Problem 7.  Exercises 1(a) and 1(b) from page 216.

Problem 8.  Exercise 1 from page 243.

Problem 9.  Set up a triple integral that represents the 3-dimensional volume of the solid region

$\displaystyle S = \left\{ (x,y,z) \in \mathbb{R}^3 : x^2+y^2+z^2 \leq 1 \right\}$.

Note: do not evaluate this triple integral.  Although it is possible to compute, it is a bit tedious (and relies on some cool integration techniques); we will learn a better way to evaluate this integral very shortly.

## Assignment 9

Group 1 Question

The set of all points $\displaystyle S = \left\{ (x,y,z) : (x-1)^2 + y^2 + \frac{z^2}{4} = 1\right\}$ forms a sphere-like object called an ellipsoid centered at the point $(1, 0, 0)$ (a picture of this surface is shown below).

Find the points on this ellipsoid that are closest to and farthest away from the point the origin.

Group 2 Question

Find the equation for the tangent plane to the surface determined by

$\displaystyle y^z + x^z + x^y = 3$

at the point $(1, 1, 1)$.

Group 3 Question

Find the maximum and minimum values of $f(x,y,z) = xyz$ over all points in the region of space $W = \left\{(x,y,z) : 2x^2+2y^2+z^2 \leq 2 \right\}$.

Group 4 Question

Find the linear approximation of the function

$\displaystyle F(u, v) = \left(\,e^u, \arctan\left(\frac{v^2}{1+u^2}\right), e^v\right)$

at the point \$latex (0, 1)4.

Group 5 Question

Let $f(x,y,z)$ be a real-valued function that satisfies

$\displaystyle \nabla f\,(1, -1, \sqrt{2}) = \langle 1, 2, -2 \rangle$

(and note that there are many functions that satisfy this).  Given this information, compute the partial derivative $\partial f/\partial \theta$ at the point with spherical coordinates $(\rho, \theta, \varphi) = (2, -\pi/4, \pi/4)$.

Group 6 Question

For this question let $A$ be a symmetric, $2\times 2$ matrix of real numbers

$\displaystyle A = \left[ \begin{array}{cc} a& b \\ b & c \end{array}\right]$

and define a function $f:\mathbb{R}^2\to\mathbb{R}$ by

$\displaystyle f(x,y) = f(\vec{x}) = \vec{x}\cdot\,A\vec{x}$.

(a) Show that the gradient of $f$ satisfies $\nabla f = 2A\vec{x}$.

(b) Compute the Hessian matrix of $f$.

(c) Show that $f$ has a critical point $\vec{x}_0 = (x_0, y_0)$ \textit{on the unit circle} precisely when

$\displaystyle A\vec{x}_0 = \lambda\,\vec{x}_0$

for some $\lambda \in \mathbb{R}$.

(d) (Bonus) What is a vector $\vec{x}_0$ that satisfies an equation like that in part (c) called?

## Vector HW 8

Problem 1.  A picture of the level set $\displaystyle S = \left\{(x,y,z) : z^3+xy^2 - x^2y - 1=0 \right\}$ is shown below.

Find the equation of the plane tangent to this surface at the point $(0, 0, 1)$.

Problem 2.  Find all points on the unit sphere $S = \{(x,y,z) : x^2+y^2+z^2=1\}$ whose tangent planes are parallel to the plane $x -y -z = 5$.

Problem 3.  (a) Compute the directional derivative of $f(x,y) = \arctan(xy+x^2)$ at the point $(1, 0)$ in the direction of $\vec{u} = \langle 0, 1 \rangle$.

(b) For the same function $f(x,y)$ as in part (a), find the direction in which $f(x,y)$ increases the most at the point $(1, 0)$.  Compute this rate of change.

Problem 4.  (a) Find all of the critical points for $f(x,y) = x^3 + xy^2$.

(b) Compute the Hessian matrix for $f(x,y)$ and use it to classify the critical points from part (a) as local minima, local maxima, saddle points, or as “unclassifiable.”

(c ) Find where the function $f(x,y)$ achieves its absolute minimum and absolute minimum on the compact set $\displaystyle D = \left\{(x,y) : x^2+y^2 \leq 1\right\}.$

## Vector HW 7

Problem 1.  Verify that the Chain Rule (as stated in class) is true for the functions

$\displaystyle F(x,y) = (x+y, ye^x) \text{ and } G(s, t) = (s+t, s-t, s^2)$.

Do this by first writing out their composition as a function $H$ and then computing the derivative matrix $DH$.

Next, compute the derivative matrices $DF$ and $DG$ and multiply them.  Your final answer should equal $DH$.

Problem 2.  Consider the function $f(x,y) = y/x$.  Use the Chain Rule to compute $\partial f/ \partial \theta$ where $\theta$ is our standard cylindrical coordinate variable.

Problem 3.  (a) Suppose $G: \mathbb{R}^3 \to \mathbb{R}^3$ is a differentiable function and that at the point $\vec{x}_0 = \left(1, 2, 1\right)$ we know that $G(\vec{x}_0) = \left(0, 0, 2\right)$ and we know that

$\displaystyle DG\big{|}_{\vec{x}_0} = \left[ \begin{array}{ccc} 1 & 2 & \pi \\ 0 & 1 & -1 \\ -1 & 7 & 1/2 \end{array} \right]$

Compute the linear approximation, $L(x,y,z)$, for $G$ at the point $\vec{x}_0$.

(b) Suppose $F:\mathbb{R}^3 \to \mathbb{R}^2$ is a differentiable function and that at the point $\vec{z}_0 = \left(0, 0, 2\right)$ we know that

$\displaystyle DF \big{|}_{\vec{z}_0} = \left[ \begin{array}{ccc} 1 & 1 & 1 \\ \sqrt{2} & -2 & 5 \end{array}\right]$.

Compute the derivative matrix $D\left(F\circ G\right)$ at the point $\vec{x}_0 = (1, 2, 1)$ where $G$ is the function from part (a).

Problem 4. Find the equation of the plane that passes through the point $\vec{x}_0 = (1, 2, 4)$ and is parallel to the plane $x - 4y + 2 = 0$.

Problem 5.  Draw a picture of the set $S = \left\{(x,y,z) \in \mathbb{R}^3 : x^2+4x + y^2 + z^2 = -3 \right\}$.

Problem 6.  Draw a picture of the line parameterized by

$\displaystyle \ell(t) = (1+t, 1+t)$.

Problem 7.  Consider the function $f(x,y) = x^3(x^2-y^2)$.

(a) Compute the derivative matrix $Df$.

(b) Compute the gradient $\nabla f$.

(c) Form the new function $g:\mathbb{R}\to\mathbb{R}$ defined by

$\displaystyle g(t) = \left( f\circ \ell\right)\,(t) = f\left(\ell(t)\right)$

where $\ell(t)$ is the function that parameterized the line in problem 6.  Note that this function $g(t)$ is the kind of function we might see in a Calculus I class; for old time’s sake, compute the derivative $g'(t)$.

(d) Use the Chain Rule to compute the $1\times 1$ derivative matrix

$\displaystyle Dg$

where $g(t)$ is the same composed function from part (c).

Note:  Your computation for part (d) could have been re-phrased in terms of the gradient of $f$ instead of its derivative matrix, $Df$.  Had we done this, we would have found

$\displaystyle g'(t) = \frac{d}{dt}\, \left(f\circ \ell\right) = \nabla f \cdot \ell'(t)$.

This is an important note!  It is our definition of something called a directional derivative!  That is, we think of this computation as telling us how $f$ changes at the point $\ell(0) = (1, 1)$ when we move inputs in the direction of $\ell$!