Explanation:
arithmetic sequence means that every new term is created by adding a certain constant (here 2) to the previous term.
a27 = -1/2 = a1 + (n-1)×2 = a1 + 26×2 = a1 + 52
a1 = -52 1/2 = -105/2
an = -105/2 + (n-1)×2
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