To simplify the expression \( 81^{-1/2} \), you can use the properties of exponents.
The expression can be rewritten as follows:
\[ 81^{-1/2} = \frac{1}{81^{1/2}} \]
Next, we calculate \( 81^{1/2} \), which is the square root of 81:
\[ 81^{1/2} = \sqrt{81} = 9 \]
Therefore, substituting this back into the expression gives:
\[ 81^{-1/2} = \frac{1}{9} \]
So, the equivalent expression to \( 81^{-1/2} \) is:
\[ \frac{1}{9} \]