下列各组数据属于勾股数的是[ ]
A．2，3，4
B．4，5，6
C．7，8，9
D．5，12，13
答案：D
——— Alice looks sad. Did you tell her about the accident? ——— Yes， but I ________ her just before her final exams.
A．shouldn’t tell
B．shouldn’t have told
C．needn’t tell
D．mustn’t have told
其中f(x)有连续二阶导数，试求
其中f(x)有连续二阶导数，试求