To describe the term \( a_7 \) in the given sequence \( 81, 27, 9, 3, 1, ... \), we first note that the sequence appears to be a geometric sequence where each term is obtained by dividing the previous term by 3.
The first five terms are:
- \( a_1 = 81 \)
- \( a_2 = 27 \)
- \( a_3 = 9 \)
- \( a_4 = 3 \)
- \( a_5 = 1 \)
To find subsequent terms:
- \( a_6 = \frac{a_5}{3} = \frac{1}{3} \)
- \( a_7 = \frac{a_6}{3} = \frac{1/3}{3} = \frac{1}{9} \)
Now we can conclude:
\[ a_7 = \frac{1}{9} \]
Therefore, the correct choice is:
a7=19 A sub 7 is equal to 1 ninth.