`\sqrt{-x^2-2x+4}=x-3` `ĐK:x >= 3`
`<=>-x^2-2x+4=x^2-6x+9`
`<=>2x^2-4x-5=0`
Ptr có: `\Delta'=(-2)^2-2.(-5)=14 > 0`
`=>` Ptr có `2` nghiệm pb
`x_1=[-b'+\sqrt{\Delta'}]/a=[2+\sqrt{14}]/2` (ko t/m)
`x_2=[-b'-\sqrt{\Delta'}]/a=[2-\sqrt{14}]/2` (ko t/m)
Vậy ptr vô nghiệm
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`\sqrt{x-3}-2\sqrt{x^2-9}=0` `ĐK: x >= 3`
`<=>\sqrt{x-3}-2\sqrt{(x-3)(x+3)}=0`
`<=>\sqrt{x-3}(1-2\sqrt{x+3})=0`
`<=>` $\left[\begin{matrix} \sqrt{x-3}=0\\ 1-2\sqrt{x+3}=0\end{matrix}\right.$
`<=>` $\left[\begin{matrix} x-3=0\\ \sqrt{x+3}=\dfrac{1}{2}\end{matrix}\right.$
`<=>` $\left[\begin{matrix} x=3(t/m)\\ x+3=\dfrac{1}{4}=\dfrac{-11}{4} (ko t/m)\end{matrix}\right.$
Vậy `S={3}`
