`a+b+c+ab+bc+ac= 9/4`

`=>ab+bc+ac = 9/4 - (a+b+c)`

`(a+b+c)^2 = a^2 +b^2 +c^2 +2(ab+bc+ac)`

`=>a^2 +b^2 +c^2 = (a+b+c)^2 - 2(ab+bc+ac)`

`=>a^2 +b^2 +c^2 = (a+b+c)^2 - 2.[9/4 - (a+b+c)]`

`=> a^2 +b^2 +c^2 = (a+b+c)^2 +2(a+b+c) - 9/2`

`=> a^2 +b^2 +c^2 = (a+b+c)^2 +2(a+b+c)+1 - 9/2 -1`

`=>a^2 +b^2 +c^2 = (a+b+c+1)^2 - 11/2 >= -11/2`

`=> K >= -11/2`

`=>K_(min) = -11/2`

Dấu "=" xảy ra `<=>a+b+c=-1`.