`a) z^2 +6z +5 -t^2 -4t`
`=z^2 +6z+9-4-t^2 -4t`
`=(z^2 +6z+9)-(t^2 +4t+4)`
`=(z+3)^2 -(t+2)^2`
`b) x^2 -2xy +2y^2 +2y+1`
`=x^2 -2xy+y^2 +y^2 +2y+1`
`=(x^2 -2xy+y^2)+(y^2 +2y+1)`
`=(x-y)^2 +(y+1)^2`
`c) 4x^2 -12x-y^2 +2y+1`
`=4x^2 -12x-y^2 +2y+9-7-1`
`=(4x^2 -12x+9)-(y^2 -2y+1)-7`
`=(2x-3)^2 -(y-1)^2 -7`
chúc học tốt