ab−a−b+1=?(a) (a−1)(b−1)(b) (1−a)(1−b)(c) (a−1)(1−b)(d) (1−a)(b−1)
1−2ab−(a2+b2)=?(a) (1+a−b)(1+a+b)(b) (1+a+b)(1−a+b)(c) (1+a+b)(1−a−b)(d) (1+a−b)(1−a+b)