`3x²+3y²+4xy+2x-2y+2=0`
⇒`9x²+9y²+12xy+6x-6y+6=0`
⇒`(3x)²+2.3x(2y+1)+9y²-6y+6=0`
⇒`(3x+2y+1)²-(2y+1)²+9y²-6y+6=0`
⇒`(3x+2y+1)²-4y²-4y-1+9y²-6y+6=0`
⇒`(3x+2y+1)²+5y²-10y+5=0`
⇒`5(3x+2y+1)²+25y²-50y+25=0`
⇒`5(3x+2y+1)²+(5y-5)²=0`
Ta co
`5(3x+2y+1)²+(5y-5)²` $\ge$ 0 `∀x`
Dấu `=` xảy ra khi `3x+2y+1=0 , 5y-5=0`
⇒ `x= -1 , y=1`
Thay `x= -1 , y=1` vào P ta đc
`P=2`