※ 引述《tzhau (生命中無法承受之輕)》之銘言:
: 設雙曲線x^2-y^2=2上有兩點A、B,
: 其兩點的x座標皆為正數
: O為原點,試求向量OA和向量OB內積之最小值
x = (1/√2) [u + v]
y = (1/√2) [-u + v]
(1/2)[2v 2u] = 2
=> uv = 1
=> OA * OB = (u_1, 1/u_1) * (u_2, 1/u_2)
= u_1 u_2 + 1/[u_1 u_2]
>= 2
=> OA * OB的最小值 = 2
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