Question

9．已知函數(shù)f（x）=$\left\{\begin{array}{l}{3x，x≤0}\\{lo{g}_{2}x，x＞0}\end{array}\right.$則f[f（$\frac{1}{2}$）]的值是（　�。�

• A． -3
• B． 3
• C． $\frac{1}{3}$
• D． -$\frac{1}{3}$

Answer 1

分析 根據(jù)函數(shù)f（x）=$\left\{\begin{array}{l}{3x，x≤0}\\{lo{g}_{2}x，x＞0}\end{array}\right.$的解析式，將x=$\frac{1}{2}$代入可得答案．

解答 解：∵函數(shù)f（x）=$\left\{\begin{array}{l}{3x，x≤0}\\{lo{g}_{2}x，x＞0}\end{array}\right.$，
∴f（$\frac{1}{2}$）=${log}_{2}\frac{1}{2}$=-1，
∴f[f（$\frac{1}{2}$）]=f（-1）=-3，
故選：A．

點(diǎn)評(píng) 本題考查的知識(shí)點(diǎn)是分段函數(shù)的應(yīng)用，函數(shù)求值，難度不大，屬于基礎(chǔ)題．