`A=(x\sqrtx-2x)/(\sqrtx-2)+(x\sqrtx+1)/(x-\sqrtx+1)` `a)` Rút gọn `b)` Tim `x` để `A > 0` với mọi `x` thuộc `R^(**)`
2 câu trả lời
Bạn tham khảo nhé.
`a,`
`A=\frac{x\sqrt{x}-2x}{\sqrt{x}-2}+\frac{x\sqrt{x}+1}{x-\sqrt{x}+1}(x\ne4,x>=0)`
`=\frac{x(\sqrt{x}-2)}{\sqrt{x}-2}+\frac{\sqrt{x}^3+1^3}{x-\sqrt{x}+1}`
`=x+\frac{(\sqrt{x}+1)(x-\sqrt{x}+1)}{x-\sqrt{x}+1}`
`=x+\sqrt{x}+1`
$\\$
`b,`
`A>0`
`=>x+\sqrt{x}+1>0`
`<=>x+2.\sqrt{x}. 1/2+1/4+3/4>0`
`<=>(\sqrt{x})^2+2.\sqrt{x}.1/2+(1/2)^2+3/4>0`
`<=>(\sqrt{x}+1/2)^2+3/4>0`
Ta có: `\sqrt{x}+1/2>=1/2AAx>=0`
`=>(\sqrt{x}+1/2)^2>=(1/2)^2=1/4AAx>=0`
`=>(\sqrt{x}+1/2)^2+3/4>=1/4+3/4=1>0AAx>=0`
`->A>0AAx>=0`
Vì ta có điều kiện `x>=0` nên `x\inRR^(+);x\ne4`