Rate Change Inverse 25Bc2D
1. **Problem 1: Find the instantaneous rate of change of** $f(x) = 3x^2 - 6x$ **at** $x=1$.
The instantaneous rate of change of a function at a point is given by the derivative evaluated at that point.
2. **Formula:**
$$f'(x) = \frac{d}{dx}(3x^2 - 6x)$$
3. **Calculate the derivative:**
$$f'(x) = 6x - 6$$
4. **Evaluate at** $x=1$:
$$f'(1) = 6(1) - 6 = 6 - 6 = 0$$
So, the instantaneous rate of change at $x=1$ is $0$.
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5. **Problem 2: Analyze the function** $f(x) = - \frac{x}{x^2 + 4}$ **and determine which statement is not true:**
- $f$ has a local maximum at $x = -2$.
- $f$ has a local minimum at $x = 2$.
- $f$ is decreasing on $(-2, 2)$.
- $f$ is increasing on $(-2, \infty)$.
6. **Find the derivative to analyze increasing/decreasing and extrema:**
$$f(x) = - \frac{x}{x^2 + 4}$$
Use quotient rule:
$$f'(x) = - \frac{(1)(x^2 + 4) - x(2x)}{(x^2 + 4)^2} = - \frac{x^2 + 4 - 2x^2}{(x^2 + 4)^2} = - \frac{-x^2 + 4}{(x^2 + 4)^2} = \frac{x^2 - 4}{(x^2 + 4)^2}$$
7. **Critical points where** $f'(x) = 0$:
$$x^2 - 4 = 0 \implies x = \pm 2$$
8. **Sign of** $f'(x)$:
- For $x$ in $(-2, 2)$, $x^2 - 4 < 0$ so $f'(x) < 0$ (decreasing).
- For $x > 2$ or $x < -2$, $f'(x) > 0$ (increasing).
9. **Check statements:**
- Local max at $x = -2$: Since $f'(x)$ changes from positive to negative at $x=-2$, this is true.
- Local min at $x = 2$: Since $f'(x)$ changes from negative to positive at $x=2$, this is true.
- $f$ is decreasing on $(-2, 2)$: True.
- $f$ is increasing on $(-2, \infty)$: False, because $f'(x) < 0$ on $(-2, 2)$ and only positive on $(2, \infty)$.
So, the false statement is: **$f$ is increasing on $(-2, \infty)$**.
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10. **Problem 3: Given relation** $R = \{(x,y): y = 2x - 3\}$ **with domain** $\{-2, -1, 1, 3\}$, find the domain of** $R^{-1}$.
11. **Calculate the range of** $R$ for given domain:
$$y = 2x - 3$$
For $x = -2$: $y = 2(-2) - 3 = -4 - 3 = -7$
For $x = -1$: $y = 2(-1) - 3 = -2 - 3 = -5$
For $x = 1$: $y = 2(1) - 3 = 2 - 3 = -1$
For $x = 3$: $y = 2(3) - 3 = 6 - 3 = 3$
12. **The inverse relation** $R^{-1}$ **swaps** $x$ **and** $y$, so the domain of $R^{-1}$ is the range of $R$:
$$\{-7, -5, -1, 3\}$$
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**Final answers:**
- Instantaneous rate of change at $x=1$ is $0$.
- The false statement about $f(x) = - \frac{x}{x^2 + 4}$ is: "$f$ is increasing on $(-2, \infty)$".
- The domain of $R^{-1}$ is $\{-7, -5, -1, 3\}$.