Choosen As Best Answer

把頭尾額外加進去:插入 n 個數,加上額外的頭尾,共有 n+2 個數
15, ..., 60,總和 = 450 + 15 + 60 = 525
S = (15 + 60)×(n + 2)÷2 = 525
75n + 150 = 1050
75n = 900, n = 12 #

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S46 - S45
= (a1 + a2 + ... + a45 + a46) - (a1 + a2 + ... + a45)
= a46
= 90 - 50 = 40 #

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