b) `\sqrt{x^2-9}+\sqrt{x-3}=0` `(x>=3)`
⇔`\sqrt{(x-3)(x+3)}+\sqrt{x-3}=0`
⇔`\sqrt{x-3}.\sqrt{x+3}+\sqrt{x+3}=0`
⇔`\sqrt{x-3}(\sqrt{x+3}+1)=0`
⇔\(\left[ \begin{array}{l}\sqrt{x-3}=0\\\sqrt{x+3}+1=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x-3=0\\\sqrt{x+3}=-1 \text{(vô lý)}\end{array} \right.\)
⇔`x-3=0`
⇔`x=3` `(tm)`
Vậy `S={3}`
c) `2/3\sqrt{9x-9}-1/4\sqrt{16x-16}+27\sqrt{(x-1)/(81)}=4` `(x>=1)`
⇔`2/3\sqrt{9(x-1)}-1/4\sqrt{16(x-1)}+27.(\sqrt{x-1})/(9)=4`
⇔`2/(3).3\sqrt{x-1}-1/(4).4\sqrt{x-1}+(27\sqrt{x-1})/(9)=4`
⇔`2\sqrt{x-1}-\sqrt{x-1}+3\sqrt{x-1}=4`
⇔`4\sqrt{x-1}=4`
⇔`\sqrt{x-1}=1`
⇔`x-1=1`
⇔`x=2` `(tm)`
Vậy `S={2}`
