Cubic Curve 02233A
1. **State the problem:** We need to sketch the curve of the function $$y = (x+3)(x+1)(x-2)$$.
2. **Formula and important rules:** This is a cubic polynomial function. To sketch it, we find its roots (where $$y=0$$), analyze its end behavior, and find critical points (where the slope is zero) to understand its shape.
3. **Find the roots:** Set $$y=0$$:
$$ (x+3)(x+1)(x-2) = 0 $$
Roots are $$x = -3, -1, 2$$.
4. **End behavior:** Since the leading term when expanded is $$x^3$$ (positive coefficient), as $$x \to \infty$$, $$y \to \infty$$, and as $$x \to -\infty$$, $$y \to -\infty$$.
5. **Find critical points:** Differentiate:
$$y = (x+3)(x+1)(x-2)$$
Use product rule:
$$y' = (x+1)(x-2) + (x+3)(x-2) + (x+3)(x+1)$$
Simplify each:
$$(x+1)(x-2) = x^2 - x - 2$$
$$(x+3)(x-2) = x^2 + x - 6$$
$$(x+3)(x+1) = x^2 + 4x + 3$$
Sum:
$$y' = (x^2 - x - 2) + (x^2 + x - 6) + (x^2 + 4x + 3) = 3x^2 + 4x - 5$$
6. **Solve for critical points:**
$$3x^2 + 4x - 5 = 0$$
Use quadratic formula:
$$x = \frac{-4 \pm \sqrt{4^2 - 4 \cdot 3 \cdot (-5)}}{2 \cdot 3} = \frac{-4 \pm \sqrt{16 + 60}}{6} = \frac{-4 \pm \sqrt{76}}{6}$$
$$\sqrt{76} \approx 8.7178$$
So,
$$x_1 = \frac{-4 - 8.7178}{6} \approx -2.7863$$
$$x_2 = \frac{-4 + 8.7178}{6} \approx 0.7863$$
7. **Find corresponding $$y$$ values:**
$$y(-2.7863) = (-2.7863+3)(-2.7863+1)(-2.7863-2) \approx (0.2137)(-1.7863)(-4.7863) \approx 1.83$$
$$y(0.7863) = (0.7863+3)(0.7863+1)(0.7863-2) \approx (3.7863)(1.7863)(-1.2137) \approx -8.21$$
8. **Summary:**
- Roots at $$x = -3, -1, 2$$
- Local maximum near $$(-2.79, 1.83)$$
- Local minimum near $$(0.79, -8.21)$$
- End behavior: $$y \to -\infty$$ as $$x \to -\infty$$ and $$y \to \infty$$ as $$x \to \infty$$
This information allows us to sketch the curve accurately.