The second and fifth terms of an arithmetic sequence are 17 and 12, respectively. What is the eighth term?
Call "R" the addition between terms
From T2 to T5 is 3R and T5 is 5 less than T2
From T5 to T8 is 3R so T8 is 5 less than T5
T8 is 12 – 5 so T8 = 7
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+1518 
