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Verified Answer :

MissPhiladelphia

In
order to solve for a nth term in an arithmetic sequence, we use the formula
written as:


an = a1 + (n-1)d


where an is the nth term, a1 is the first value
in the sequence, n is the term position and d is the common difference.


First, we need to calculate for d from the given
values above.  
a3 = 20.5 and a8 = 13



an = a1 + (n-1)d

20.5 = a1 + (3-1)d

an = a1 + (n-1)d
13 = a1 + (8-1)d

a1 = 23.5
d = -1.5


The 11th term is calculated as follows:


a11 = a1 + (n-1)d

a11= 23.5 + (11-1)(-1.5)
a11 =
8.5

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