✨ 最佳解答 ✨
首項 = a = -5
S9 - S6 = a7 + a8 + a9
= a+6d + a+7d + a+8d
= 3a + 21d = 69
21d = 69 - 3a = 69 - 3 × (-5) = 84
=> d = 4 #
✨ 最佳解答 ✨
首項 = a = -5
S9 - S6 = a7 + a8 + a9
= a+6d + a+7d + a+8d
= 3a + 21d = 69
21d = 69 - 3a = 69 - 3 × (-5) = 84
=> d = 4 #
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