a, 3x-$\sqrt[7]{x}$ + 4 =0
<=> 3x-$\sqrt[3]{x}$ -$\sqrt[4]{x}$ +4 =0
<=> (3x-$\sqrt[3]{x}$) - ($\sqrt[4]{x}$ +4) =0
<=> $\sqrt[3]{x}$ ($\sqrt{x}$ -1) - 4($\sqrt{x}$ -1) = 0
<=> ($\sqrt{x}$ -1)($\sqrt[3]{x}$ -4) =0
<=> \(\left[ \begin{array}{l}\sqrt{x} -1\\\sqrt[3]{x} -4\end{array} \right.\) =0
b,
ĐKXĐ x > 1/2
<=>2x - 3√(2x - 1) - 5 = 0
<=> 2x - 1 - 3√(2x - 1) - 4 = 0
<=> [√(2x - 1) - 4][√(2x - 1) + 1] = 0
<=> √(2x - 1) - 4 = 0 (Do √(2x - 1) + 1 > 0 với mọi x > 1/2)
<=> √(2x - 1) = 4
<=> 2x - 1 = 16
<=> x = 17/2 (TM)
