Answer
3. Statement of Problem: Given $y′=3+2y$, a) draw the associated direction field: b) determine the behavior of $y$ as $t →∞$; c) if the behavior depends on the initial value of $y$ at $t=0$, describe the dependency.
Answer:
a) the direction field has an equilibrium solution of $y=-(\frac{3}{2})$. Some solutions for $y′=3+2y$ are: $y′(0)=3$, $y′(1)=5$, $y′(-1)=1$, $y'(-\frac{3}{2})=0$, and $y′(-2)=-1$ .
b) As $t →∞$, the behavior of y depends upon the initial conditions (the value of $y_0$).
c) There are three possible behaviors of $y$:
If $y_0=-\frac{3}{2}$, then the value of y does not change; $y=-\frac{3}{2}$ for all $t$. As $t →∞$, $y→-\frac{3}{2}$.
If $y_0\gt-\frac{3}{2}$, then as $t →∞$, $y→∞$.
If $y_0\lt-\frac{3}{2}$, then as $t →∞$, $y→-∞$.
Work Step by Step
Note: $y'$ is a function of $y$. In essence, $y′=y′(y)$. Additionally, $y=y(t)$. Directional fields of differential equations give a value for $\frac{dy}{dt}$ for a set of values of $y$ and $t$.
a) Start graphing a directional field by finding any equilibrium solutions. To find the equilibrium solution (called $y_{eq}$) of a first order differential equation $y′=a+by$, let $y′=0$, then solve the right side of the original equation for $y$. This gives the equilibrium solution $y_{eq}=-\frac{a}{b}$. In a graph of $y$ vs. $t$, this will appear as a horizontal line; any equation $y(t)$ that "touches" this line for a given value of $t$ will continue along the line for all greater values of $t$. Since $y'$ does not depend on $t$, $y'$ will be the same at a given value of $y$ for all values of t. Find some values of $y'$ for values of $y$. Some sample answers are given above: "Some solutions for $y′=3+2y$ are: $y′(0)=3$, $y′(1)=5$, $y′(-1)=1$, $y'(-\frac{3}{2})=0$, and $y′(-2)=-1$." Graphing all of the values here, along with the equilibrium solution $y_{eq}$, should give the directional field enough information to see the general behavior of $y$. REMEMBER that for the given equation, all values of $t$ for a given $y$ have the same value for $y′$.
b) Pick several starting points: some below, some above, and some along $y_{eq}$. Trace an approximate graph of each particular $y$, based on the values of $y'$that were calculated, and observe the behavior of each as $t →∞$. The behavior of y should be apparent: it will either approach $∞$, $-∞$, or it will approach a value equal to one of the equilibrium values.
c)The behavior of the function $y(t)$ depends on $y_0$.
If $y_0=-\frac{3}{2}$, then initial $y'=0$. The rate of change in $y$ is zero, so $y_0=y$ will be for all values of $t$.
If $y_0\gt-\frac{3}{2}$, then as $t →∞$, $y→∞$. For all of these values of $y$, $y'\gt0$. Because of this, as $t$ increases, $y$ will continuously increase.
If $y_0\lt-\frac{3}{2}$, then as $t →∞$, $y→-∞$. For all of these values of $y$, $y'\lt0$. Because of this, as $t$ increases, $y$ will continuously decrease.
