<p><strong>72.</strong> Find the value of \(4^5 \log_8(3-\sqrt{6}) - 6 \log_8(\sqrt{3}-\sqrt{2})\).</p>
Step-by-Step Solution
Key Concept: Simplify the arguments of logarithms by recognizing that 3−√6 and √3−√2 can be expressed as perfect squares or products, allowing us to use logarithm properties to reduce the expression.
Step 1: Simplify the argument $3-\sqrt{6}$.
Observe that:
$$3 - \sqrt{6} = (\sqrt{3})^2 - \sqrt{2}\sqrt{3} = \sqrt{3}(\sqrt{3} - \sqrt{2})$$
Step 2: Apply logarithm properties.
Using the simplification from Step 1, we have:
$$\log_8(3-\sqrt{6}) = \log_8[\sqrt{3}(\sqrt{3}-\sqrt{2})] = \log_8(\sqrt{3}) + \log_8(\sqrt{3}-\sqrt{2})$$
Step 3: Substitute into the original expression.
Substitute the result from Step 2 into the given expression:
$$4^5 \log_8(3-\sqrt{6}) - 6 \log_8(\sqrt{3}-\sqrt{2})$$
$$= 4^5[\log_8(\sqrt{3}) + \log_8(\sqrt{3}-\sqrt{2})] - 6 \log_8(\sqrt{3}-\sqrt{2})$$
$$= 4^5 \log_8(\sqrt{3}) + 4^5 \log_8(\sqrt{3}-\sqrt{2}) - 6 \log_8(\sqrt{3}-\sqrt{2})$$
$$= 4^5 \log_8(\sqrt{3}) + (4^5 - 6) \log_8(\sqrt{3}-\sqrt{2})$$
Step 4: Calculate $4^5$.
$$4^5 = (2^2)^5 = 2^{10} = 1024$$
Step 5: Evaluate the expression.
Substitute the value of $4^5$ into the expression from Step 3:
$$1024 \log_8(\sqrt{3}) + (1024 - 6) \log_8(\sqrt{3}-\sqrt{2})$$
$$= 1024 \log_8(3^{1/2}) + 1018 \log_8(\sqrt{3}-\sqrt{2})$$
$$= 1024 \cdot \frac{1}{2} \log_8(3) + 1018 \log_8(\sqrt{3}-\sqrt{2})$$
$$= 512 \log_8(3) + 1018 \log_8(\sqrt{3}-\sqrt{2})$$
Correct Answer: 0