  X X2 2 2P X X X 2 22       2 2 2 2X X X X2 1X   1PVẬY

Bài 4. Với , ta cĩ:   x x2 2 2P x x x 2 22

   

  

  2 2 2 2x x x x2 1x   1PVậy . 0; 4aa