Đáp án+Giải thích các bước giải:
`12) x^2 - y^2 - x + y`
`= (x^2 - y^2) - (x - y)`
`= (x - y)(x + y) - (x - y)`
`= (x - y)[(x + y) - 1]`
`= (x - y)(x + y - 1)`
----------------------------
`13) 3x^2 (a + b + c) + 108y^2 ( a + b + c)`
`= (a + b + c) .(3x^2 + 108 y^2)`
`= 3.(a + b + c).(x^2 + 36y^2)`
----------------------------
`14) x^4 + x^3 + x + 1`
`= (x^4 + x^3) + (x + 1)`
`= x^3 (x + 1) + (x + 1)`
`= (x + 1)(x^3 + 1)`
`= (x + 1)(x^3 + 1^3)`
`= (x + 1)(x + 1)(x^2 - x + 1)`
`= (x + 1)^2 (x^2 - x + 1)`
----------------------------
`15) x^2 y + xy^2 - x - y`
`= (x^2 y + xy^2) - (x + y)`
`= xy(x + y) - (x + y)`
`= (x + y)(xy - 1)`
----------------------------
`16) x^4 - x^3 - x^2 + 1`
`= (x^4 - x^3) - (x^2 - 1)`
`= x^3 (x - 1) - (x^2 - 1^2)`
`= x^3 (x - 1) - (x - 1)(x + 1)`
`= (x - 1)[x^3 - (x + 1)]`
`= (x - 1)(x^3 - x - 1)`
`#SunHee`
