The 18th term is 98.
An arithmetic sequence is a sequence of numbers in which the difference of any two successive members is a constant. This sequence starts with -21, and each subsequent number increases by 7.
The formula for the nth term of an arithmetic sequence is:
a_n = a_1 + (n - 1) * d
where:
- a_n is the nth term we're trying to find
- a_1 is the first term in the sequence
- d is the common difference
- n is the position of the term in the sequence
Let's plug in the given values into the formula:
First term, a_1 = -21
Common difference, d = 7
We're looking for the 18th term, so n = 18
Now, we can calculate:
a_18 = -21 + (18 - 1) * 7
Doing the operations in parentheses first:
a_18 = -21 + 17 * 7
Then multiplication:
a_18 = -21 + 119
And lastly, addition:
a_18 = 98
So, the 18th term of the given arithmetic sequence is 98.