`a) \sqrt{x}=6` ĐK:`x>=0`
`<=> x=6^2`
`<=> x=36(\text{tm})`
Vậy `S={36}`
`b) \sqrt{x}-2=0` ĐK:`x>=0`
`<=> \sqrt{x}=2`
`<=> x=4(\text{tm})`
Vậy `S={4}`
`c) x^2=25`
`<=> x^2-25=0`
`<=> (x-5)(x+5)=0`
`<=> [(x-5=0),(x+5=0):}`
`<=> [(x=5),(x=-5):}`
Vậy `S={+-5}`
`d) x^2-6=0`
`<=> (x-\sqrt{6})(x+\sqrt{6})=0`
`<=> [(x-\sqrt{6}=0),(x+\sqrt{6}=0):}`
`<=> [(x=\sqrt{6}),(x=-\sqrt{6}):}`
Vậy `S={+-\sqrt{6}}`