Có : C = [(x^2+2xy+y^2)-2.(x+y)+1] + (9y^2 + 8y + 16/9) + 155/9
= (x+y-1)^2 + (3y+4/3)^2 + 155/9 >= 155/9
Dấu "=" xảy ra <=> x+y-1 = 0 và 3y+4/3 =0
<=> x= 13/9 ; y= -4/9
\(C=\left(x^2+9y^2+1+2xy-2x-6y\right)+\left(y^2+12y+36\right)-17\)
\(=\left(x+y-1\right)^2+\left(y+6\right)^2-17\ge-17\)
Dấu "=" xảy ra \(\Leftrightarrow x=7;y=-6\)
Vậy min C=.....................
\(C=\left(x^2+9y^2+1+2xy-2x-6y\right)+\left(y^2+12y+36\right)_{ }_{ }-17\)
\(=\left(x+3y-1\right)^2+\left(y+6\right)^2-17\ge-17\)
dấu "=" xảy ra khi x=19,y=-6