Logarithms
System of Logarithmic Equations with Exponential Form
GRB_1000_MCQ
Grade Class 12
Question:
Let $\alpha, \beta, \gamma$ are positive real numbers such that $\log_\gamma(2\alpha) = \dfrac{1}{3}$, $\log_\gamma(5\beta) = \dfrac{1}{6}$ and $\log_\gamma(\alpha\beta) = \dfrac{3}{2}$, then:
$\alpha^3 = \dfrac{1}{16}$
$\alpha^3 = \dfrac{1}{80}$
$\gamma = \dfrac{1}{10}$
$\beta^{12} = \dfrac{5^{14}}{2}$
Step-by-Step Solution
Step 1: Convert the logarithmic equations to exponential form.
From the definition of logarithms, $\log_b a = c \iff b^c = a$.
$$ \log_\gamma(2\alpha) = \frac{1}{3} \implies 2\alpha = \gamma^{1/3} \quad (1) $$
$$ \log_\gamma(5\beta) = \frac{1}{6} \implies 5\beta = \gamma^{1/6} \quad (2) $$
$$ \log_\gamma(\alpha\beta) = \frac{3}{2} \implies \alpha\beta = \gamma^{3/2} \quad (3) $$
Step 2: Express $\alpha$ and $\beta$ in terms of $\gamma$.
From equation (1), $\alpha = \frac{\gamma^{1/3}}{2}$.
From equation (2), $\beta = \frac{\gamma^{1/6}}{5}$.
Step 3: Substitute the expressions for $\alpha$ and $\beta$ into equation (3) to solve for $\gamma$.
$$ \left(\frac{\gamma^{1/3}}{2}\right) \left(\frac{\gamma^{1/6}}{5}\right) = \gamma^{3/2} $$
$$ \frac{\gamma^{1/3 + 1/6}}{10} = \gamma^{3/2} $$
$$ \frac{\gamma^{2/6 + 1/6}}{10} = \gamma^{3/2} $$
$$ \frac{\gamma^{3/6}}{10} = \gamma^{3/2} $$
$$ \frac{\gamma^{1/2}}{10} = \gamma^{3/2} $$
Since $\gamma$ is a positive real number, we can divide by $\gamma^{1/2}$:
$$ \frac{1}{10} = \frac{\gamma^{3/2}}{\gamma^{1/2}} $$
$$ \frac{1}{10} = \gamma^{3/2 - 1/2} $$
$$ \frac{1}{10} = \gamma^1 $$
Thus, $\gamma = \frac{1}{10}$.
Step 4: Calculate $\alpha^3$.
Substitute $\gamma = \frac{1}{10}$ into the expression for $\alpha$:
$$ \alpha = \frac{\gamma^{1/3}}{2} = \frac{(1/10)^{1/3}}{2} $$
Now, cube both sides:
$$ \alpha^3 = \left(\frac{(1/10)^{1/3}}{2}\right)^3 = \frac{(1/10)^{(1/3) \cdot 3}}{2^3} = \frac{1/10}{8} = \frac{1}{80} $$
Step 5: Calculate $\beta^{12}$.
Substitute $\gamma = \frac{1}{10}$ into the expression for $\beta$:
$$ \beta = \frac{\gamma^{1/6}}{5} = \frac{(1/10)^{1/6}}{5} $$
Now, raise both sides to the power of 12:
$$ \beta^{12} = \left(\frac{(1/10)^{1/6}}{5}\right)^{12} = \frac{(1/10)^{(1/6) \cdot 12}}{5^{12}} = \frac{(1/10)^2}{5^{12}} = \frac{1/100}{5^{12}} $$
$$ \beta^{12} = \frac{1}{100 \cdot 5^{12}} = \frac{1}{(10^2) \cdot 5^{12}} = \frac{1}{(2 \cdot 5)^2 \cdot 5^{12}} = \frac{1}{2^2 \cdot 5^2 \cdot 5^{12}} = \frac{1}{2^2 \cdot 5^{14}} = \frac{1}{4 \cdot 5^{14}} $$
Correct Answer: 1, 3, 4