Chapter 14 - Sequences, Series, and the Binomial Theorem - 14.2 Arithmetic Sequences and Series - 14.2 Exercise Set - Page 902: 24

$a_{20}=-83$

The n th term of an arithmetic sequence can be calculated as: $a_n=a_1+d\times(n-1)$ Here: $n=20$, $d=-5$ and we have to find $a_{20}$. By substituting we get: $a_{20}=12+(-5)\times(20-1)=12-95=-83$ $a_{20}=-83$