To find the 28th term ($a_{28}$) of the given arithmetic sequence, we use the formula for the $n$-th term of an arithmetic sequence:

$a_n = a_1 + (n-1)d$

Given:

• $a_1 = 22$ (the first term),
• $d = -1$ (common difference, calculated as $21 - 22 = -1$),
• $n = 28$ (term number to find).

Substituting into the formula:

$a_{28} = 22 + (28-1)(-1)$ $a_{28} = 22 + 27(-1)$ $a_{28} = 22 - 27$ $a_{28} = -5$

Final Answer:

$a_{28} = -5$

Would you like a detailed explanation or further clarification? Here are 5 related questions you might find interesting:

1. What is the sum of the first 28 terms of this sequence?
2. How do we determine the common difference in an arithmetic sequence?
3. Can we derive the general formula for any term in this sequence?
4. What happens to the sequence if the common difference changes to $+1$?
5. How does the formula for the $n$-th term differ for a geometric sequence?

Tip: Always verify the common difference before applying the arithmetic sequence formula.