ab−a−b+1=?(a) (a−1)(b−1)(b) (1−a)(1−b)(c) (a−1)(1−b)(d) (1−a)(b−1)
If the coefficient of x7 in [ax2+(1bx)]11 equals the coefficient of x−7 in [ax2−(1bx)]11, then 'a' and 'b' satisfy the relation