$$ (64)^{-\frac{1}{2}} - (-32)^{-\frac{4}{5}} = $$ $x$
Aptitude
Surds and Indices
Difficulty: Medium
Choose an option
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A$$ \frac{1}{8} $$
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B$$ \frac{3}{8} $$
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C$$ \frac{1}{16} $$
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D$$ \frac{3}{16} $$
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ENone of these
Answer
Correct Answer: $$ \frac{1}{16} $$
Explanation
Concept & Formula
This question combines the evaluation of fractional exponents with the application of negative bases.
$$ (x^m)^{\frac{1}{n}} = (x^{\frac{1}{n}})^m $$
$$ a^{-n} = \frac{1}{a^n} $$
Step-by-Step Solution
* Evaluate the first term: $$ (64)^{-\frac{1}{2}} $$
Since $$ 64 = 8^2 $$, apply the power rule:
$$ (8^2)^{-\frac{1}{2}} = 8^{-1} = \frac{1}{8} $$
* Evaluate the second term: $$ (-32)^{-\frac{4}{5}} $$
Recognize that $$ -32 = (-2)^5 $$. Apply the power rule:
$$ ((-2)^5)^{-\frac{4}{5}} = (-2)^{-4} $$
* Convert the negative exponent into a fraction:
$$ (-2)^{-4} = \frac{1}{(-2)^4} $$
Since any negative number raised to an even power becomes positive:
$$ (-2)^4 = 16 $$
So, the second term simplifies to $$ \frac{1}{16} $$.
* Substitute both simplified terms back into the main expression:
$$ \frac{1}{8} - \frac{1}{16} $$
* Find a common denominator (16) and subtract:
$$ \frac{2}{16} - \frac{1}{16} = \frac{1}{16} $$
Exam Strategy & Shortcut
Break down complex terms independently. For the second term, immediately note the denominator of the exponent is 5, meaning you need a 5th root. The 5th root of -32 is exactly -2. Then, raise -2 to the remaining numerator power, which is -4. So, it becomes $$(-2)^{-4}$$, yielding $$1/16$$. The first term is simply the reciprocal of the square root of 64, giving $$1/8$$. Simply calculate $$1/8 - 1/16 = 1/16$$ instantly.
Common Pitfall
The most dangerous pitfall here is mishandling the negative base in the second term. Students often see the negative sign in $$-32$$ and incorrectly carry it forward to make the final fraction negative, completely forgetting that raising a negative number to an even power (the 4 in the exponent numerator) makes the result positive.
Final Answer
**Therefore, the correct answer is $$ \frac{1}{16} $$.**