`text{Bài 1}`
`D = (5; +infty)`
`log_{3} (x - 3) + log_{3} (x - 5) = 1`
`-> log_{3} [(x - 3)(x - 5)] = 1`
`-> (x - 3)(x - 5) = 3^1 = 3`
`-> x^2 - 5x - 3x + 15 = 3`
`-> x^2 - 8x + 12 = 0`
`->` \(\left[ \begin{array}{l}x=6\\x=2 (l)\end{array} \right.\)
`-> x = 2`
`-> text{Chọn A}`
`text{Bài 2}`
`D = (2; +infty)`
$log_{\dfrac{1}{2}} (x - 2) + log_{2}(x + 2) = 1$
`-> log_{2^{-1}} (x - 2) + log_{2} (x + 2) = 1`
`-> log_{2} (x + 2) - log_{2} (x - 2) = 1`
`-> log_{2} ((x + 2)/(x - 2)) = 1`
`-> (x + 2)/(x - 2) = 2^1 = 2`
`-> x + 2 = 2x - 4`
`-> x = 6`
`-> text{Chọn B}`
`text{Bài 47}`
`text{Ta có}`
`2^{x} + 2^{x + 1} + 2^{x + 2} = 21`
`-> 2^{x}(1 + 2^{1} + 2^{2}) = 21`
`-> 2^{x}.7 = 21`
`-> 2^{x} = 3`
`-> x = log_{2} 3`
`-> text{Chọn C}`
