题目内容
14.当x为何值时,$\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}}}$才有意义.分析 根据偶次被开方数大于等于零、真数大于零列出不等式组,根据对数的运算求解.
解答 解:要使函数有意义,则$\left\{\begin{array}{l}{x>0}\\{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}}}\end{array}\right.≥0$,
所以$\left\{\begin{array}{l}{x>0}\\{\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}}}\end{array}\right.≥1$,则$\left\{\begin{array}{l}{x>0}\\{lg\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}}\end{array}\right.≥1$,即$\left\{\begin{array}{l}{x>0}\\{\sqrt{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}}\end{array}\right.≥10$,
依次得:$\left\{\begin{array}{l}{x>0}\\{lg\sqrt{lg\sqrt{lg\sqrt{lgx}}}}\end{array}\right.≥100$,…,
解得$x≥1{0}^{1{{0}^{2}}^{1{{0}^{2}}^{1{{0}^{2}}^{1{0}^{2}}}}}$,
所以函数的定义域是[$1{0}^{1{{0}^{2}}^{1{{0}^{2}}^{1{{0}^{2}}^{1{0}^{2}}}}}$,+∞).
点评 本题考查对数函数的定义域,以及对数的运算,属于中档题.
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