Giải các phương trình sau:a) \(\log \left( {x + 1} \right) = 2;\) b) \(2{\log _4}x + {\log _2}\left( {x - 3} \right) = 2;\) c) \(\ln x + \ln \left( {x - 1} \right) = \ln 4x;\) d) \({\log _3}\left( {{x^2} - 3x + 2} \right) = {\log _3}\left( {2x - 4}...
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Giải các phương trình sau:
a) \(\log \left( {x + 1} \right) = 2;\)
b) \(2{\log _4}x + {\log _2}\left( {x - 3} \right) = 2;\)
c) \(\ln x + \ln \left( {x - 1} \right) = \ln 4x;\)
d) \({\log _3}\left( {{x^2} - 3x + 2} \right) = {\log _3}\left( {2x - 4} \right).\)
a, ĐK: \(x+1>0\Leftrightarrow x>-1\)
\(log\left(x+1\right)=2\\ \Leftrightarrow x+1=10^2\\ \Leftrightarrow x+1=100\\ \Leftrightarrow x=99\left(tm\right)\)
b, ĐK: \(\left\{{}\begin{matrix}x-3>0\\x>0\end{matrix}\right.\Rightarrow x>3\)
\(2log_4x+log_2\left(x-3\right)=2\\ \Leftrightarrow log_2x+log_2\left(x-3\right)=2\\ \Leftrightarrow log_2\left(x^2-3x\right)=2\\ \Leftrightarrow x^2-3x=4\\ \Leftrightarrow x^2-3x-4=0\\ \Leftrightarrow\left(x+1\right)\left(x-4\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-1\left(ktm\right)\\x=4\left(tm\right)\end{matrix}\right.\)
c, ĐK: \(x>1\)
\(lnx+ln\left(x-1\right)=ln4x\\ \Leftrightarrow ln\left[x\left(x-1\right)\right]-ln4x=0\\ \Leftrightarrow ln\left(\dfrac{x-1}{4}\right)=0\\ \Leftrightarrow\dfrac{x-1}{4}=1\\ \Leftrightarrow x-1=4\\ \Leftrightarrow x=5\left(tm\right)\)
d, ĐK: \(\left\{{}\begin{matrix}x^2-3x+2>0\\2x-4>0\end{matrix}\right.\Rightarrow x>2\)
\(log_3\left(x^2-3x+2\right)=log_3\left(2x-4\right)\\ \Leftrightarrow x^2-3x+2=2x-4\\ \Leftrightarrow x^2-5x+6=0\\ \Leftrightarrow\left(x-2\right)\left(x-3\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=2\left(ktm\right)\\x=3\left(tm\right)\end{matrix}\right.\)