Stationary Inflection
1. **State the problem:** Given the function $g(x) = x^3 - 3x^2 - 9x - 5$, we are asked to find the first and second derivatives, verify stationary points, determine their nature, find the inflection point, and sketch the graph with key features.
2. **Find $g'(x)$ and $g''(x)$:**
- Differentiate $g(x)$ term-by-term:
$$g'(x) = 3x^2 - 6x - 9$$
- Differentiate $g'(x)$ to get $g''(x)$:
$$g''(x) = 6x - 6$$
3. **Show that the stationary points are $(-1,0)$ and $(3,-32)$:**
- Stationary points occur where $g'(x) = 0$:
$$3x^2 - 6x - 9 = 0$$
- Divide entire equation by 3:
$$x^2 - 2x - 3 = 0$$
- Factor:
$$(x - 3)(x + 1) = 0$$
- So stationary points at $x = 3$ and $x = -1$.
- Evaluate $g(-1)$:
$$(-1)^3 - 3(-1)^2 - 9(-1) - 5 = -1 - 3 + 9 - 5 = 0$$
- Evaluate $g(3)$:
$$3^3 - 3(3)^2 - 9(3) - 5 = 27 - 27 - 27 - 5 = -32$$
4. **Determine the nature of each stationary point:**
- Use $g''(x)$:
- At $x = -1$:
$$g''(-1) = 6(-1) - 6 = -6 - 6 = -12 < 0$$
So $(-1,0)$ is a local maximum.
- At $x = 3$:
$$g''(3) = 6(3) - 6 = 18 - 6 = 12 > 0$$
So $(3,-32)$ is a local minimum.
5. **Find the inflection point:**
- Inflection point occurs where $g''(x) = 0$:
$$6x - 6 = 0 \,\Rightarrow\, x = 1$$
- Evaluate $g(1)$:
$$1^3 - 3(1)^2 - 9(1) - 5 = 1 - 3 - 9 - 5 = -16$$
- Inflection point is at $(1, -16)$.
6. **Graph features summary:**
- Stationary points: $(-1, 0)$ local max and $(3, -32)$ local min.
- Inflection point: $(1, -16)$.
- X-intercepts at $(-1, 0)$ and $(5, 0)$.
Final answer:
- $$g'(x) = 3x^2 - 6x - 9$$
- $$g''(x) = 6x - 6$$
- Stationary points: $(-1,0)$ (max), $(3,-32)$ (min)
- Inflection point: $(1,-16)$