1. **State the problem:** We want to analyze the function $f(x) = x \sqrt{-2x + 5}$.
2. **Domain:** The expression inside the square root must be non-negative for real values, so:
$$-2x + 5 \geq 0$$
$$-2x \geq -5$$
Dividing both sides by $\cancel{-2}$ (remember to flip the inequality sign because we divide by a negative):
$$x \leq \frac{5}{2}$$
3. **Function behavior:** The function is defined for all $x$ such that $x \leq \frac{5}{2}$.
4. **Simplify the function:** Write $f(x)$ explicitly:
$$f(x) = x \sqrt{-2x + 5}$$
5. **Find critical points:** To find maxima or minima, differentiate $f(x)$ using the product and chain rules.
Let $g(x) = x$ and $h(x) = \sqrt{-2x + 5} = (-2x + 5)^{1/2}$.
Then:
$$f'(x) = g'(x)h(x) + g(x)h'(x)$$
Calculate derivatives:
$$g'(x) = 1$$
$$h'(x) = \frac{1}{2}(-2x + 5)^{-1/2} \cdot (-2) = -\frac{1}{(-2x + 5)^{1/2}}$$
So:
$$f'(x) = 1 \cdot (-2x + 5)^{1/2} + x \cdot \left(-\frac{1}{(-2x + 5)^{1/2}}\right) = \sqrt{-2x + 5} - \frac{x}{\sqrt{-2x + 5}}$$
6. **Set derivative to zero to find critical points:**
$$\sqrt{-2x + 5} - \frac{x}{\sqrt{-2x + 5}} = 0$$
Multiply both sides by $\sqrt{-2x + 5}$:
$$(-2x + 5) - x = 0$$
$$-2x + 5 - x = 0$$
$$-3x + 5 = 0$$
$$3x = 5$$
$$x = \frac{5}{3}$$
7. **Check if $x=\frac{5}{3}$ is in the domain:** Since $\frac{5}{3} \approx 1.666 \leq 2.5$, it is valid.
8. **Evaluate $f(x)$ at critical point:**
$$f\left(\frac{5}{3}\right) = \frac{5}{3} \sqrt{-2 \cdot \frac{5}{3} + 5} = \frac{5}{3} \sqrt{-\frac{10}{3} + 5} = \frac{5}{3} \sqrt{\frac{5}{3}} = \frac{5}{3} \cdot \frac{\sqrt{15}}{3} = \frac{5 \sqrt{15}}{9}$$
9. **Summary:** The function $f(x) = x \sqrt{-2x + 5}$ is defined for $x \leq \frac{5}{2}$, has a critical point at $x = \frac{5}{3}$ where it attains a local maximum of $\frac{5 \sqrt{15}}{9}$.
Final answer: The domain is $(-\infty, \frac{5}{2}]$ and the local maximum is at $x=\frac{5}{3}$ with value $f\left(\frac{5}{3}\right) = \frac{5 \sqrt{15}}{9}$.
Function Analysis 9Dec09
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