a. 2 √x-1=12
⇔ √x-1= 6
⇔ x-1 =±36
⇔ \(\left[ \begin{array}{l}x-1=36\\x-1=-36\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=37\\x=-35\end{array} \right.\)
b. 2 (√x+3)-5 ≤7
⇔ 2√x + 6 - 5 ≤7
⇔ 2√x +1 ≤7
⇔ 2√x ≤ 6
⇔ √x ≤ 3
⇔ x ≤ ±9
c. 4 √x> √x+2
⇔ 4√x - √x > 2
⇔ 3√x > 2
⇔ √x > $\frac{2}{3}$
⇔ x > ± $\frac{4}{9}$
d. 2 √x+39=35
⇔ 2√x = -4
⇔ √x = -2 ( vô lý )
⇒ x vô nghiệm