Question

find the 18th term of this arithmetic sequence.
-21, -14, -7, 0, 7, ...
$a_{18} = ?$
hint: $a_n = a_1 + (n - 1)d$

Explanation:

Step1: Identify known values

$a_1 = -21$, $n=18$, $d = -14 - (-21) = 7$

Step2: Substitute into arithmetic formula

$a_{18} = a_1 + (n-1)d = -21 + (18-17)\times7$

Step3: Simplify the expression

$a_{18} = -21 + 17\times7 = -21 + 119$

Step4: Calculate final value

$a_{18} = 98$

Answer:

$98$