a) ĐKXĐ: \(x\ge-\dfrac{1}{3}\)
\(\sqrt{3x+1}=7\\ \Leftrightarrow3x+1=49\\ \Leftrightarrow x=16\)
b) \(3\sqrt{9x-27}-\dfrac{1}{5}\sqrt{25x-75}-\dfrac{1}{7}\sqrt{49x-147}=21\\ \Leftrightarrow9\sqrt{x-3}-\sqrt{x-3}-\sqrt{x-3}=21\\ \Leftrightarrow7\sqrt{x-3}=21\\ \Leftrightarrow\sqrt{x-3}=3\\ \Leftrightarrow x-3=9\\ \Leftrightarrow x=12\)
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