Family Solutions Aa819C
1. **Problem Statement:** We are given a differential equation $y' = f(y)$ and a graph of $y = f(y)$.
2. **Goal:** Sketch a representative family of solutions for the differential equation based on the graph of $f(y)$.
3. **Key Idea:** The slope of the solution curve $y(x)$ at any point depends only on the value of $y$ at that point, since $y' = f(y)$.
4. **Step-by-step analysis:**
- Identify equilibrium points where $f(y) = 0$. From the graph, these occur approximately at $y \approx -1$, $y \approx 0$, and $y \approx 0.6$.
- Determine stability of equilibria:
- At $y \approx -1$, $f(y)$ crosses from positive to negative, so this is a stable equilibrium.
- At $y \approx 0$, $f(y)$ crosses from negative to positive, so this is an unstable equilibrium.
- At $y \approx 0.6$, $f(y)$ crosses from positive to negative, so this is a stable equilibrium.
- For $y$ values between equilibria, the sign of $f(y)$ tells us whether solutions increase or decrease:
- For $y$ between $-1$ and $0$, $f(y) < 0$, so $y$ decreases.
- For $y$ between $0$ and $0.6$, $f(y) > 0$, so $y$ increases.
5. **Sketching solutions:**
- Solutions starting near $y = -1$ tend to stay near $-1$ (stable).
- Solutions starting just above $-1$ decrease toward $-1$.
- Solutions starting between $-1$ and $0$ decrease toward $-1$.
- Solutions starting just below $0$ increase away from $0$ (unstable).
- Solutions starting between $0$ and $0.6$ increase toward $0.6$.
- Solutions starting above $0.6$ decrease toward $0.6$.
6. **Summary:** The family of solutions will show horizontal asymptotes at $y = -1$ and $y = 0.6$, with solutions moving away from the unstable equilibrium at $y=0$.
This qualitative analysis allows sketching solution curves without explicit formulas.