Đáp án:
`a. x^2+4x-y^2+4`
`= (x^2+4x+4)-y^2`
`= (x+2)^2-y^2`
`= (x+2-y)(x+2+y)`
`b. 25-4x^2-4xy-y^2`
`= 5^2-(4x^2+4xy+y^2)`
`= 5^2-(2x+y)^2`
`= (5-2x-y)(5+2x+y)`
`c. x^2-2xy+y^2-z^2+2zt-t^2`
`= (x^2-2xy+y^2)-(z^2-2zt+t^2)`
`= (x-y)^2-(z-t)^2`
`= (x-y-z+t)(x-y+z-t)`