`a,`
`-x^2 + 324 = 0`
`⇔ -x^2 = -324`
`⇔ x^2 = 324`
`⇔ x = \pm\sqrt{324}`
`⇔ x = \pm 18`
Vậy `S = {18,-18}`
`b,`
`16x^2 - 5 = 0`
`⇔ 16x^2 = 5`
`⇔ x^2 = 5/16`
`⇔ x = \pm\sqrt{5/16}`
`⇔ x = \pm\sqrt{5}/4`
Vậy `S ={\sqrt{5}/4 , -\sqrt{5}/4}`
`c,`
`2/(\sqrt{x}-3) = 4`
`⇔ 2 = 4(\sqrt{x}-3)`
`⇔ 2 = 4\sqrt{x}-12`
`⇔ 4\sqrt{x}-12=2`
`⇔ 4\sqrt{x}=14`
`⇔ \sqrt{x}= 7/2`
`⇔ x = 49/4`
Vậy `S = {49/4}`
