a) `3/2\sqrt{4x-8}-9\sqrt{(x-2)/(81)}=6` `(x>=2)`
⇔`3/2\sqrt{4(x-2)}-9(\sqrt{x-2})/(\sqrt{81})=6`
⇔`3\sqrt{x-2}-(9\sqrt{x-2})/(9)=6`
⇔`3\sqrt{x-2}-\sqrt{x-2}=6`
⇔`2\sqrt{x-2}=6`
⇔`\sqrt{x-2}=3`
⇔`x-2=9`
⇔`x=11` `(tm)`
Vậy `S={11}`
b) `\sqrt{9x-9}-\sqrt{4x-4}+\sqrt{16x-16}-3\sqrt{x-1}=16` `(x>=1)`
⇔`\sqrt{9(x-1)}-\sqrt{4(x-1)}+\sqrt{16(x-1)}-3\sqrt{x-1}=16`
⇔`3\sqrt{x-1}-2\sqrt{x-1}+4\sqrt{x-1}-3\sqrt{x-1}=16`
⇔`2\sqrt{x-1}=16`
⇔`\sqrt{x-1}=8`
⇔`x-1=64`
⇔`x=65` `(tm)`
Vậy `S={65}`
