# Chapter 11 - Section 11.2 - Arithmetic Sequences - Exercise Set - Page 839: 27

$a_n=7-4(n-1)$; $a_{20} = -69$

#### Work Step by Step

RECALL: (1) The $n^{th}$ term, $a_n$, of an arithmetic sequence can be found using the formula: $a_n=a_1 + d(n-1)$ where $a_1$ = first term $n$ = term number $d$ = common difference (2) The common difference $d$ can be determined using the formula $d= a_n-a_{n-1}$ where $a_n$ = $n^{th}$ term $a_{n-1}$ = term before $a_n$ Solve for the common difference using formula (2) above to obtain: $d =3-7 \\d = -4$ The given arithmetic sequence has $a_1=7$ and $d=-4$. Substitute these values into the formula in (1) above to obtain the arithmetic sequence's formula for the general term: $a_n=7+(-4)(n-1) \\a_n=7-4(n-1)$ Solve for the 20th term for the sequence using the formula above to obtain: $a_{20} = 7 - 4(20-1) \\a_{20} = 7-4(19) \\a_{20} = 7-76 \\a_{20} = -69$

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