Bài 30. Tìm x, biết:
a) \({\log _5}x = 4;\) b) \({\log _2}\left( {5 - x} \right) = 3;\)
c) \({\log _3}\left( {x + 2} \right) = 3;\) d) \({\log _{{1 \over {16}}}}\left( {0,5 + x} \right) = - 1;\)
a) \({\log _5}x = 4 \Leftrightarrow x = {5^4} = 625.\)
b) \({\log _2}\left( {5 - x} \right) = 3 \Leftrightarrow 5 - x = {2^3} \Leftrightarrow x = - 3\);
c) \({\log _3}\left( {x + 2} \right) = 3 \Leftrightarrow x + 2 = {3^3} \Leftrightarrow x = 25\);
d) \({\log _{{1 \over {6}}}}\left( {0,5 + x} \right) = - 1 \Leftrightarrow 0,5 + x = {\left( {{1 \over {6}}} \right)^{ - 1}} \Leftrightarrow x = 6 - 0,5 = 5,5\).
Copyright © 2021 HOCTAP247