To simplify \( 81^{-1/2} \), we can apply the rules of exponents.
The expression \( 81^{-1/2} \) can be rewritten as:
\[ \frac{1}{81^{1/2}} \]
Since \( 81^{1/2} \) is the square root of 81, we have:
\[ 81^{1/2} = \sqrt{81} \]
Thus, we can write:
\[ 81^{-1/2} = \frac{1}{\sqrt{81}} \]
Now, we can evaluate:
\[ \sqrt{81} = 9 \]
So,
\[ 81^{-1/2} = \frac{1}{9} \]
From the options given, the expression \( \frac{1}{\sqrt{81}} \) is the equivalent expression we derived.
Therefore, the correct response is:
1/√81