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111學測數學A考科-02


<單選題>某品牌計算機在計算對數 \( \log_a b \) 時需按 \( \boxed{\log} \boxed{a} \boxed{b} \)。某生在計算 \( \log_a b \) 時(其中 \( a \gt 1 \) 且 \( b \gt 1 \))順序弄錯,誤按 \( \boxed{\log} \boxed{b} \boxed{a} \),所得為正確值的 \( \frac{9}{4} \) 倍。試選出 a,b 間的關係式。
(1) \( a^2 = b^3 \) (2) \( a^3 = b^2 \) (3) \( a^4 = b^9 \) (4) \( 2a = 3b \) (5) \( 3a = 2b \)

答案

由題意得 \( \frac{\log b}{\log a} = \frac{9}{4} \cdot \frac{\log a}{\log b} \Rightarrow (\log a)^2 = \frac{9}{4} (\log b)^2 \Rightarrow \log a = \frac{3}{2} \log b \)
∴ \( a = b^{\frac{3}{2}} \Rightarrow a^2 = b^3 \)
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