阅读下面两首古诗，完成后面的题。
                                                                                                           《十五夜望月》                        《望洞庭》
                                                                                                                      王建                                     刘禹锡
                                                                                                         中庭地白树栖鸦，                     湖光秋月两相和，
                                                                                                         冷露无声湿桂花。                     潭面无风镜未磨。
                                                                                                         今夜月明人尽望，                     遥望洞庭山水色，
                                                                                                         不知秋思落谁家？                     白银盘里一青螺。

已知函数f（x）=$\frac{1}{3}$x³+2x²﹣ax+1在（﹣1，1）上存在极值点，则实数a的取值集合为____