题目

Susan's mother looks so young. It's hard to imagine she's already in her ______. A . sixty B . sixties C . sixtieth 答案：B已知函数f（x）=x2+a，（x∈R）．（1）对?x1，x2∈R比较与的大小；（2）若x∈[-1，1]时，有|f（x）|≤1，试求实数a的取值范围．