Question

Answer 1

`@` Xét `[2x]/[x^2+1] >= 1`

`<=>[2x-x^2-1]/[x^2+1] >= 0`

  Mà `x^2+1 > 0`

  `=>2x-x^2-1 >= 0`

`<=>x=1`

   `=>A={1}`

`@` Xét `x^2-2bx+4=0` vô nghiệm `<=>\Delta' < 0`

           `<=>(-b)^2-4 < 0<=>b^2 < 4<=>-2 < b < 2`

_________________________________________

  `=>A nn B={1}`

      `->\bb A`