Đáp án:
`B = ab(a + b) - bc(b + c) + ca(a - c)`
`= ab[(a - c) + (b + c)] - bc(b + c) + ca(a - c)`
`= [ab(a - c) + ca(a - c)] + [ab(b + c) - bc(b + c)]`
`= (a - c)(ab + ca) + (b + c)(ab - bc)`
`= (a - c)(b + c) a + (b + c)(a - c)b`
`= (a - c)(b + c)(a + b)`
`C = ab(a + b) + bc(b + c) + ca(c + a) + 3abc`
`= ab(a + b) + bc(b + c) + ca(c + a) + abc + abc + abc`
`= [ab(a + b) + abc] + [bc(b + c) + abc] + [ca(c + a) + abc]`
`= ab(a + b + c) + bc(b + c + a) + ca(a + b + c)`
`= (a + b + c)(ab + bc + ca)`
