等差数列中,d=3/4,a28=57/4,则a1=?

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a(n) = a + 3(n-1)/4.
57/4 = a(28) = a + 3*27/4,
a(1) = a = 57/4 - 3*27/4 = [57- 81]/4 = -24/4 = -6

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