9/13/2014 The 4^th term of an arithmetic series is 16 whereas the 13^th term is 43. Find the 20^th term.

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9/13/2014 Solution.
Since the 4^th term of the arithmetic series is 16 and the 13^th term is 43,

a₁₃ = a₄ + 9d → 43 = 16 + 9d → d = 3

We need the 20^th term, so that a₂₀ = a₄ + 16d = 16 + 16·3 = 64