How do you find $a_{21}$ $a_21$ given 121, 118, 115, ...?

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How do you find $a_{21}$ $a_21$ given 121, 118, 115, ...?

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Answer 1 · 2016-12-11T20:21:42

If $a (1) = 121$ $a(1)=121$ , $a (2) = 118$ $a(2)=118$ , and $a (3) = 115$ $a(3)=115$ , we can create an equation for $a (n)$ $a(n)$ , then plug in $n = 21$ $n=21$ to get $a (21)$ $a(21)$ .

$a (n) = - 3 n + b$ $a(n)=-3n+b$
$121 = - 3 (1) + b$ $121=-3(1)+b$
$b = 124$ $b=124$

$a (n) = - 3 n + 124$ $a(n)=-3n+124$

$a (21) = - 3 (21) + 124$ $a(21)=-3(21)+124$
$= 61$ $=61$

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How do you find $a_{21}$ $a_21$ given 121, 118, 115, ...?

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How do you find a21a_21 given 121, 118, 115, ...?

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How do you find $a_{21}$ $a_21$ given 121, 118, 115, ...?