`C(x)=2x^2+3y^2+4xy-8x-2y+18`
`=2x^2+4xy+2y^2-8x-8y+y^2+6y+18`
`=2(x+y)^2-8(x+y)+8+y^2+6y+9+1`
`=2(x+y-2)^2+(y+3)^1+1>=1`
Dấu "=" `<=>y=-3,x=5`
`d)D(x)=2x^2+3y^2+4z^2-2(x+y+z)+2`
`=2x^2-2x+1/2+3y^2-2y+1/3+4z^2-2z+1/4+11/12`
`=2(x-1/2)^2+3(y-1/3)^2+(2z-1)^2+11/12>=11/12`
Dấu "=" `<=>x=1/2,y=1/3,z=1/4`
`E(x)=2x^2+8xy+11y^2-4x-2y+6`
`=2x^2+8xy+8y^2-4x-8y+3y^2+6y+6`
`=2(x+2y)^2-4(x+2y)+2+3y^2+6y+3+1`
`=2(x+y-1)^2+3(y+1)^2+1>=1`
Dấu "=" `<=>y=-1,x=2`