Answer: Therefore, the correct formula is {a}_{21} = 2000 * 0.5^{20}.
Explanation:
A geometric sequence is a sequence of numbers in which each term after the first is found by multiplying the previous term by a constant factor called the common ratio. In this case, the common ratio is 0.5, since each term is found by dividing the previous term by 2.
To find the nth term in a geometric sequence, we can use the formula:
a_n = a_1 * r^(n-1)
where a_1 is the first term in the sequence, a_n is the nth term, and r is the common ratio. In our case, the first term is 2000, the nth term is the 21st term, and the common ratio is 0.5. Therefore, the formula we need to use is:
a_21 = 2000 * 0.5^(21-1)
= 2000 * 0.5^20
Therefore, the correct formula is {a}_{21} = 2000 * 0.5^{20}.