Question

[Mot So]

giải các bất phương trình sau:

1) \(\dfrac{x^2-2x+5}{x-2}-x+1\ge0\)                2) \(\dfrac{2x-3}{x+1}-2< 0\)

Answer 1

[Thanh Quân]

1) \(ĐK:x\ne2\) 

Nếu \(x>2\) 

BPT ⇔ \(x^2-2x+5-\left(x-1\right)\left(x-2\right)\ge0\) ⇔ \(x^2-2x+5-\left(x^2-3x+3\right)\ge0\)

⇔\(x+2\ge0\) ⇔\(x\ge-2\) ⇒ Lấy \(x\ge2\)

Nếu \(x< 2\)

BPT ⇔\(\dfrac{-\left(x^2-2x+5\right)}{x-2}-x+1\ge0\) ⇔\(-x^2+2x-5-\left(x-1\right)\left(x-2\right)\ge0\)

⇔\(-x^2+2x-5-x^2+3x-2\ge0\)

⇔\(-2x^2+5x-7\ge0\)

⇔\(x^2-\dfrac{5}{2}x+\dfrac{7}{2}\le0\)

⇔\(\left(x-\dfrac{5}{4}\right)^2\le\dfrac{11}{4}\)

⇔\(\left[{}\begin{matrix}x-\dfrac{5}{4}\le\dfrac{11}{4}\\x-\dfrac{5}{4}\le\dfrac{-11}{4}\end{matrix}\right.\) ⇔\(\left[{}\begin{matrix}x\le4\\x\le\dfrac{-3}{2}\end{matrix}\right.\) ⇔ \(x\le\dfrac{-3}{2}\) 

S= [2;+∞)U(-∞;\(\dfrac{-3}{2}\)]

Answer 2

[Thanh Quân]

2) \(ĐK:x\ne-1\) 

Nếu \(x>-1\) 

BPT ⇔ \(2x-3-2\left(x+1\right)< 0\) ⇔\(2x-3-2x-2< 0\)

 ⇔\(-5< 0\) ( luôn đúng với mọi \(x>-1\))

Nếu \(x< -1\)

BPT⇔\(\dfrac{-\left(2x-3\right)}{x+1}-2< 0\) ⇔\(-\left(2x-3\right)-2\left(x+1\right)< 0\) ⇔\(-4x+1< 0\) ⇔ \(x>\dfrac{-1}{4}\)

Vậy S=....