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$\#phnn$
`a)\sqrt(x-3)=\sqrt(x^2-5x+6)` `\text{ĐK:}` `x\ge3`
`<=>x-3=x^2-5x+6`
`<=>x-3-x^2+5x-6=0`
`<=>6x-x^2-9=0`
`<=>-(x^2-6x+9)=0`
`<=>-(x^2-2.x.3+3^2)=0`
`<=>-(x-3)^2=0`
`<=>x-3=0`
`<=>x=3`
`\text{Vậy S={3}}`
`\sqrt{x-3}=\sqrt{x^2-5x+6}` `(x>=3)`
⇔`x-3=x^2-5x+6`
⇔`-x^2+x+5x-3-6=0`
⇔`-x^2+6x-9=0`
⇔`-(x^2-6x+9)=0`
⇔`-(x-3)^2=0`
⇔`x-3=0`
⇔`x=3` `(tm)`
Vậy `S={3}`
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