[解題] 高一 數學 等比數列
1.年級：高一
2.科目：數學
3.章節：等比數列
4.題目：四數成等比 第一、四數和為28 第二、三數和為12 求公比
5.想法：
設四數為 a, ar, ar^2, ar^3
=> a(r^3+1) = 28 ...A
a(r^2+r) = 12 ...B
A/B
=> (r+1)(r^2-r+1) 7
──────── = ─ ...C
(r+1) r 3
=> (r^2-r+1)/r = 7/3
=> ...
=> r = 3 or 1/3
照這樣算是沒問題
但是學生說沒學過立方和公式...即C式左半的分子分解
請問各位先進有其他的方法可解此題嗎
感謝XD
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