If the first term of an arithmetic sequence equals the third minus the second, and the fourth term is 18, what is the first term?
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a = c - b
d = 18
a - a - b - a - c - a - 18
18 - 3a = a
18 = 4a
a = 4.5
a, b, c, d
4.5, 9, 13.5, 18
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