Math CentralQuandaries & Queries


Question from Justin, a student:

Which expression can be used to find the nth term in this sequence?
My choices are
A. -19n=113 B. -21n+115
C. -19+75 D. 21n+73

Please explain how to get the correct answer. I think it is A. but I am not sure,

Hi Justin,

I think the choices should be

A. $-19n+113$ B. $-21n+115$
C. $19n+75$ D. $21n+73$

I tried $n = 1$ for each of the four expressions and got

A. $-19 \times 1 +113 = 94$ B. $-21 \times 1 +115 = 94$
C. $19 \times 1 +75 = 94$ D. $21 \times 1+73 =94$

Thus all four expressions yield $94$ when $n = 1.$ Next try $n = 2.$ Does each expression give $75?$ If any expression doesn't yield $75$ then that expression isn't correct.




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