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\begin{document}
\section{Ma 116 Lecture 1/31/00}
\subsection{Vector Functions}
SNB uses the convention of writing a vector $v_{1}\vec{i}+v_{2}\vec{j}+v_{3}%
\vec{k}$ in the form $\left( v_{1},v_{2},v_{3}\right) .$ We shall use these
notations interchangeably.
Consider now a particle $P$ moving in $x,y,z-$space along a curve $C.$
\begin{center}
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\vspace{1pt}
\end{center}
If $\ \vec{r}$ is the vector from the origin to the point $\left( x\left(
t\right) ,y\left( t\right) ,z\left( t\right) \right) $ where the particle is
at time $t,$ then
\vspace{1pt}
\begin{equation*}
\vec{r}\left( t\right) =x\left( t\right) \vec{i}+y\left( t\right) \vec{j}%
+z\left( t\right) \vec{k}
\end{equation*}
We say that $\vec{r}\left( t\right) $ is a vector function of $t.$
\vspace{1pt}
More generally, if $\vec{V}=\vec{V}\left( t\right) $ is a vector function of
$t,$ then
\vspace{1pt}
\begin{center}
\begin{equation*}
\vec{V}\left( t\right) =V_{1}\left( t\right) \vec{i}+V_{2}\left( t\right)
\vec{j}+V_{3}\left( t\right) \vec{k}
\end{equation*}
\end{center}
\vspace{1pt}
We say the $\vec{V}\left( t\right) $ is \emph{continuous} if $V_{1}\left(
t\right) ,V_{2}\left( t\right) ,$ and $V_{3}\left( t\right) $ are all
continuous. If $V_{1}\left( t\right) ,V_{2}\left( t\right) ,$ and $%
V_{3}\left( t\right) $ are all differentiable, then we define the derivative
of $\vec{V}$ by
\vspace{1pt}
\begin{equation*}
\dfrac{d\vec{V}}{dt}=\dfrac{dV_{1}}{dt}\vec{i}+\dfrac{dV_{2}}{dt}\vec{j}+%
\dfrac{dV_{3}}{dt}\vec{k}
\end{equation*}
If $V_{1}\left( t\right) ,V_{2}\left( t\right) ,$ and $V_{3}\left( t\right) $
are all integrable, then we define the integral of $\vec{V}$ \ by
\vspace{1pt}
\begin{equation*}
\int \vec{V}\left( t\right) dt=\int V_{1}\left( t\right) \vec{i}+\int
V_{2}\left( t\right) \vec{j}+\int V_{3}\left( t\right) \vec{k}
\end{equation*}
\vspace{1pt}
Remark: $\vec{r}\left( t\right) =x\left( t\right) \vec{i}+y\left( t\right)
\vec{j}+z\left( t\right) $\vspace{1pt}$\vec{k}$ is called the position
vector of a curve $C:x\left( t\right) ,y\left( t\right) ,z\left( t\right) .$
Let us consider $\vec{r}$ at $t$ and at a nearby subsequent point $t+\Delta
t.$
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\vspace{1pt}
Note that $\dfrac{\Delta \vec{r}}{\Delta t}$ has the same direction as $%
\Delta \vec{r}$ since $\Delta t>0.$ Thus as $\Delta t\rightarrow 0,$ $\dfrac{%
\Delta \vec{r}}{\Delta t}\rightarrow \dfrac{d\vec{r}}{dt}$ and this is the
direction of the tangent vector to $C$ at any point $\left( x\left( t\right)
,y\left( t\right) ,z\left( t\right) \right) .$
\begin{example}
At what point or points is the tangent to the curve $%
C:x=t^{3},y=5t^{2},z=10t $ perpendicular to the direction of the tangent
when $t=1.$
\end{example}
We graph the curve first. $\left[
\begin{array}{ccc}
t^{3} & 5t^{2} & 10t%
\end{array}%
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We may represent $C$ by the vector $\vec{r}(t)=t^{3}\vec{i}+5t^{2}\vec{j}+10t%
\vec{k}.$ Then the tangent to $C$ at any point is
\vspace{1pt}
\begin{equation*}
\frac{d\vec{r}}{dt}=3t^{2}\vec{i}+10t\vec{j}+10\vec{k}
\end{equation*}
\vspace{1pt}
At $t=1$ the tangent is $\vec{W}=3\vec{i}+10\vec{j}+10\vec{k}.$ To find
where $\dfrac{d\vec{r}}{dt}$ is perpendicular to $\vec{W}$ we set the dot
product of these vectors equal to $0.$
\vspace{1pt}$\frac{d\vec{r}}{dt}\cdot \vec{W}=\left( 3t^{2},10t,10\right)
\cdot \left( 3,10,10\right) =\allowbreak 100+9t^{2}+100t=0$, Solution is: $%
\left\{ t=-10\right\} ,\left\{ t=-\frac{10}{9}\right\} .$ Thus at the points
$\left( -1000,500,-100\right) $ and $\left( \frac{-1000}{729},\frac{500}{81},%
\frac{-100}{9}\right) $ the tangent is perpendicular to its direction at $%
\left( 1,5,10\right) .$
\vspace{1pt}
If $\vec{U}$ and $\vec{V}$ are two vector functions of $t$ that are both
differentiable, then it is not hard to show that
\vspace{1pt}
\begin{eqnarray*}
\dfrac{d}{dt}\left( \vec{U}\cdot \vec{V}\right) &=&\frac{d\vec{U}}{dt}\cdot
\vec{V}+\vec{U}\cdot \frac{d\vec{V}}{dt} \\
\frac{d}{dt}\left( \vec{U}\times \vec{V}\right) &=&\frac{d\vec{U}}{dt}\times
\vec{V}+\vec{U}\times \frac{d\vec{V}}{dt} \\
\frac{d}{dt}\left[ \vec{U}\cdot \left( \vec{V}\times \vec{W}\right) \right]
&=&\frac{d\vec{U}}{dt}\cdot \left( \vec{V}\times \vec{W}\right) +\vec{U}%
\cdot \left( \dfrac{d\vec{V}}{dt}\times \vec{W}\right) +\vec{U}\cdot \left(
\vec{V}\times \dfrac{d\vec{W}}{dt}\right)
\end{eqnarray*}
\vspace{1pt}
If $\vec{U}=\left( u_{1},u_{2},u_{3}\right) ,\vec{V}=\left(
v_{1},v_{2},v_{3}\right) ,$ and $\vec{W}=\left( w_{1},w_{2},w_{3}\right) ,$
then since
\vspace{1pt}
\begin{equation*}
\vec{U}\cdot \left( \vec{V}\times \vec{W}\right) =\left|
\begin{array}{ccc}
u_{1} & u_{2} & u_{3} \\
v_{1} & v_{2} & v_{3} \\
w_{1} & w_{2} & w_{3}%
\end{array}%
\right|
\end{equation*}
\vspace{1pt}
the last equation above leads to the following interesting formula for the
derivative of a $3\times 3$ determinant.
\vspace{1pt}
\begin{equation*}
\dfrac{d}{dt}\left|
\begin{array}{ccc}
u_{1} & u_{2} & u_{3} \\
v_{1} & v_{2} & v_{3} \\
w_{1} & w_{2} & w_{3}%
\end{array}%
\right| =\left|
\begin{array}{ccc}
\frac{du_{1}}{dt} & \frac{du_{2}}{dt} & \frac{du_{3}}{dt} \\
v_{1} & v_{2} & v_{3} \\
w_{1} & w_{2} & w_{3}%
\end{array}%
\right| +\left|
\begin{array}{ccc}
u_{1} & u_{2} & u_{3} \\
\frac{dv_{1}}{dt} & \frac{dv_{2}}{dt} & \frac{dv_{3}}{dt} \\
w_{1} & w_{2} & w_{3}%
\end{array}%
\right| +\left|
\begin{array}{ccc}
u_{1} & u_{2} & u_{3} \\
v_{1} & v_{2} & v_{3} \\
\frac{dw_{1}}{dt} & \frac{dw_{2}}{dt} & \frac{dw_{3}}{dt}%
\end{array}%
\right|
\end{equation*}
\vspace{1pt}
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\subsection{Arc Length in Three Dimensions}
Let $C$ $:\left( x\left( t\right) ,y\left( t\right) ,z\left( t\right)
\right) $ $a\leq t\leq b$ be a curve in three space and suppose that $C$ is
\emph{smooth.} By this it is meant that $x^{\prime }\left( t\right)
,y^{\prime }\left( t\right) ,$ and $z^{\prime }\left( t\right) $ are
continuous and not all simultaneously zero on $a