Minimize Linear Expression 8Dd824
1. **State the problem:** Given the equation $\log_3 x + \log_3 y = 2$, find the minimum value of $2x + 3y$.
2. **Use logarithm properties:** Recall that $\log_a b + \log_a c = \log_a (bc)$. So,
$$\log_3 x + \log_3 y = \log_3 (xy) = 2.$$
This implies
$$xy = 3^2 = 9.$$
3. **Rewrite the problem:** We want to minimize
$$2x + 3y$$
subject to the constraint
$$xy = 9.$$
4. **Express one variable in terms of the other:** From $xy=9$, we get
$$y = \frac{9}{x}.$$
5. **Substitute into the expression to minimize:**
$$2x + 3y = 2x + 3 \cdot \frac{9}{x} = 2x + \frac{27}{x}.$$
6. **Minimize the function:** Let
$$f(x) = 2x + \frac{27}{x},$$
where $x > 0$ (since $\log_3 x$ is defined only for $x>0$).
7. **Find critical points by differentiation:**
$$f'(x) = 2 - \frac{27}{x^2}.$$
Set $f'(x) = 0$:
$$2 - \frac{27}{x^2} = 0 \implies 2 = \frac{27}{x^2} \implies x^2 = \frac{27}{2} \implies x = \sqrt{\frac{27}{2}} = \frac{3\sqrt{6}}{2}.$$
8. **Find corresponding $y$:**
$$y = \frac{9}{x} = \frac{9}{\frac{3\sqrt{6}}{2}} = \frac{9 \cdot 2}{3\sqrt{6}} = \frac{6}{\sqrt{6}} = \sqrt{6}.$$
9. **Calculate minimum value:**
$$2x + 3y = 2 \cdot \frac{3\sqrt{6}}{2} + 3 \cdot \sqrt{6} = 3\sqrt{6} + 3\sqrt{6} = 6\sqrt{6}.$$
**Final answer:** The minimum value of $2x + 3y$ given $\log_3 x + \log_3 y = 2$ is
$$\boxed{6\sqrt{6}}.$$