`a) \sqrt{x^2}=4`
`<=> |x|=4`
`<=>`\(\left[ \begin{array}{l}x=4\\x=-4\end{array} \right.\)
Vậy `S={+-4}`
`b) \sqrt{x^2}=|-6|`
`<=> |x|=6`
`<=>`\(\left[ \begin{array}{l}x=6\\x=-6\end{array} \right.\)
Vậy `S={+-6}`
`c) \sqrt{5x^2}=|-15|`
`<=> \sqrt{5}.|x|=15`
`<=> |x|=15/\sqrt{5}`
`<=> |x|=3\sqrt{5}`
`<=>`\(\left[ \begin{array}{l}x=3\sqrt{5}\\x=-3\sqrt{5}\end{array} \right.\)
Vậy `S={+-3\sqrt{5}}`
`d) \sqrt{2x+1}=x-1` ĐKXĐ: `x>=1`
`<=> 2x+1=(x-1)^2`
`<=> 2x+1=x^2-2x+1`
`<=> x^2-4x=0`
`<=> x(x-4)=0`
`<=>`\(\left[ \begin{array}{l}x=0(\text{ktm})\\x=4(\text{tm})\end{array} \right.\)
Vậy `S={4}`
