Choosen As Best Answer

前 n 項和 = -2 n^2 + 3n (⊙ t^2 = t平方)
S1 = a1 = -2 + 3 = 1
a5 = S5 - S4 = (-50 + 15) - (-32 + 12) = -15
a1 + a5 = -14 #

\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

1 ~ 10層共有 1 + 2 + ... + 9 + 10 = 55個數字
a1 = 2, a2 = 4, a3 = 6, ..., a55 = ?

最後一個數字 = 55 × 2 = 110
(2 + 110) × 55 ÷ 2 = 3080 #

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