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The flow lines (or streamlines) of a vector field are the paths followed by a particle whose velocity field is the given vector field. Thus the vectors in a vector field are tangent to the flow lines. If parametric equations of a flow line are x = x(t), y = y(t) explain why these functions satisfy the differential equations dx/dt=x and dy/dt=-y. Then solve the differential equations to find an equation of the flow line that passes through the point (1, 1).

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(b) Let x=x(t)x=x(t) and y=y(t)y=y(t) the parametric equations of a flow line. By definition of the flow lines, the velocity vector of the flow line at the point (x,y)(x, y) is is equal to the given vector field.

As the velocity vector is given by v=dx(t)/dt,dy(t)/dt\displaystyle \mathbf{v}=\langle d x(t)/ d t, d y(t) / d t \rangle, then we have v=F(x,y)\mathbf{v}=\mathbf{F}(x,y) or

dx(t)/dt,dy(t)/dt=F(x,y)=x,y\begin{equation} \langle d x(t)/ d t, d y(t) / d t \rangle=\mathbf{F}(x,y)=\langle x, -y\rangle \end{equation}

Therefore, the stream lines satisfy the differential equations

dx/dt=x,dy/dt=y\begin{equation} d x / d t=x, \qquad d y / d t=-y \end{equation}

and

dy/dt=y\begin{equation} d y / d t=-y \end{equation}

To solve (2) we write it as

dx/x=dtd x / x=d t

Hence, integrating we obtain

lnx=t+A\ln |x|=t+A

or

x=±et+A=eAet=C1et\begin{equation} x=\pm e^{t+A}=e^{A} e^{t}=C_{1}e^t \end{equation}

where AA and C1C_{1} are constants.

We can solve (3) in a similar way to obtain

y=C2et\begin{equation} y=C_{2} e^{-t} \end{equation}

where C2C_{2} is a constant.

Taking the product between (4) and (5) we have

xy=C1etC2et=C1C2=Kx y=C_{1} e^{t} C_{2} e^{-t}=C_{1}C_{2}=K

with KK a constant.

Therefore, the equations for the flow lines are

y=Kx\begin{equation} y=\frac{K}{x} \end{equation}

If the flow line passes through (1,1) we obtain from (6)

(1)=K1(1)=\frac{K}{1}

which gives K=1K=1. Then, the flow line through this point has equation

y=1x,x>0y=\frac{1}{x},\quad x>0

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