因为有a, b两个未知数，所以只需要用到两个约束。约束1：x = 1 处：右极限 = 0 = 在x = 1处的值 = (1+a+b)/2 = 0; ==> a+b = -1 .....(1)约束2: x = -1处：左极限 = 0 = 在x = -1处的值 = （-1+a-b)/2 = 0; ==> a-b = 1 ......(2)(1)(2)两式联立求得：a = 0, b = -1
