Use the formula for the general term (the nth term) of an arithmetic sequence to find the indicated term of the sequence with the given first term, a1, and common difference, d.
-$$a _ { 1 } = - 9 , d = - 0.5$$
A) $a _ { n } = - 9 n + 8.5 ; a _ { 20 } = - 171.5$
B) $a _ { n } = - 0.5 n - 9 ; a _ { 20 } = - 19$
C) $a _ { n } = - 0.5 n - 8.5 ; a _ { 20 } = - 18.5$
D) $a _ { n } = - 9 n - 0.5 ; a _ { 20 } = - 180.5$

Question

Quizplus · Accepted Answer

The formula for the nth term of an arithmetic sequence is $a_n = a_1 + (n - 1)d$. Substituting $a_1 = -9$ and $d = -0.5$ gives $a_n = -9 + (n - 1)(-0.5) = -9 - 0.5n + 0.5 = -0.5n - 8.5$. For $n = 20$, $a_{20} = -0.5(20) - 8.5 = -10 - 8.5 = -18.5$.

Use the Formula for the General Term (The Nth Term) a1=−9,d=−0.5a _ { 1 } = - 9 , d = - 0.5a1​=−9,d=−0.5

Use the Formula for the General Term (The Nth Term) $a _ { 1 } = - 9 , d = - 0.5$