Logarithms
Logarithmic Equations
GRB_1000_MCQ
Grade Class 12
Question:
Let $x = \alpha$ is a root of the equation $\log_3(9 \cdot 2^x + 9) \cdot \log_3(2^x + 1) = \log_{\frac{1}{\sqrt{3}}}\left(\dfrac{1}{\sqrt{27}}\right)$, then $\alpha$ is less than:
Step-by-Step Solution
Step 1: Simplify the right-hand side.
$$\log_{\frac{1}{\sqrt{3}}}\left(\frac{1}{\sqrt{27}}\right) = \frac{\log(1/\sqrt{27})}{\log(1/\sqrt{3})} = \frac{-\frac{3}{2}\log 3}{-\frac{1}{2}\log 3} = 3$$
Step 2: Let $t = 2^x$. The equation becomes:
$$\log_3(9t + 9) \cdot \log_3(t + 1) = 3$$
$$\log_3[9(t+1)] \cdot \log_3(t+1) = 3$$
$$(\log_3 9 + \log_3(t+1)) \cdot \log_3(t+1) = 3$$
$$(2 + \log_3(t+1)) \cdot \log_3(t+1) = 3$$
Step 3: Let $u = \log_3(t+1)$. Then:
$$(2 + u) \cdot u = 3$$
$$u^2 + 2u - 3 = 0$$
$$(u+3)(u-1) = 0$$
$$u = 1 \text{ or } u = -3$$
Step 4: Solve for $x$.
- If $u = 1$: $\log_3(2^x + 1) = 1 \Rightarrow 2^x + 1 = 3 \Rightarrow 2^x = 2 \Rightarrow x = 1$
- If $u = -3$: $\log_3(2^x + 1) = -3 \Rightarrow 2^x + 1 = 3^{-3} = \frac{1}{27} \Rightarrow 2^x = \frac{1}{27} - 1 < 0$, which is impossible.
Step 5: Therefore $\alpha = 1$. Since $\alpha = 1$, it is less than $2$, $3$, and $4$.
Correct Answer: 2, 3, 4