Stationary Points Inflection
1. **Problem statement:** Given the function $$g(x) = x^3 - 3x^2 - 9x - 5,$$ we will find its derivatives, stationary points, nature of stationary points, point of inflection, and sketch the graph labeling key points.
2. **Find the first and second derivatives:**
$$g'(x) = \frac{d}{dx}(x^3 - 3x^2 - 9x - 5) = 3x^2 - 6x - 9$$
$$g''(x) = \frac{d}{dx}(3x^2 - 6x - 9) = 6x - 6$$
3. **Find stationary points:** Stationary points occur where $$g'(x) = 0$$
Solve:
$$3x^2 - 6x - 9 = 0$$
Divide both sides by 3:
$$x^2 - 2x - 3 = 0$$
Factor:
$$(x - 3)(x + 1) = 0$$
So stationary points at $$x = 3$$ and $$x = -1$$
4. **Find coordinates of stationary points:**
Calculate $$g(x)$$ at $$x = -1$$:
$$g(-1) = (-1)^3 - 3(-1)^2 - 9(-1) - 5 = -1 - 3 + 9 - 5 = 0$$
So stationary point at $$(-1, 0)$$
Calculate $$g(3)$$:
$$g(3) = 3^3 - 3(3)^2 - 9(3) - 5 = 27 - 27 - 27 - 5 = -32$$
So stationary point at $$(3, -32)$$
5. **Determine the nature of stationary points:**
Evaluate $$g''(x)$$ at each stationary point:
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.
6. **Find the inflection point:** Inflection points occur where $$g''(x) = 0$$
Solve:
$$6x - 6 = 0$$
$$6x = 6$$
$$x = 1$$
Calculate $$g(1)$$:
$$g(1) = 1^3 - 3(1)^2 - 9(1) - 5 = 1 - 3 - 9 - 5 = -16$$
Inflection point at $$(1, -16)$$
7. **Summary of key points:**
- Stationary points: $$(-1, 0)$$ (local max), $$(3, -32)$$ (local min)
- Point of inflection: $$(1, -16)$$
- x-intercepts given at $$(-1, 0)$$ and $$(5, 0)$$
8. **Graph sketch instructions:**
Plot $g(x)$ on Cartesian plane, marking:
- Stationary points: $$(-1,0)$$ and $$(3,-32)$$
- Inflection point: $$(1, -16)$$
- x-intercepts: $$(-1,0)$$ and $$(5,0)$$
Graph rises to local max at $$(-1, 0)$$, dips to local min at $$(3, -32)$$, and passes through the inflection point $$(1, -16)$$ where concavity changes.
Final answer:
Stationary points: $$(-1,0), (3,-32)$$
Nature: $$(-1,0)$$ local max, $$(3,-32)$$ local min
Inflection point: $$(1, -16)$$