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How do you find #a_21# given 121, 118, 115, ...?

1 Answer

Dec 11, 2016

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

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

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

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

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