设，试问在点(0，0)处f(0，y)是否连续偏导数是否存在
参考答案：
解析：故f(x，y)在(0，0)处不连续．
I haven' t seen Mary these past few days. I m afraid she ____ herself for some time.
A．isn't feeling
B．hasn' t been feeling
C．hadn't been feeling
D．wasn't feeling
子女和父母的亲情，可以因家庭状况、父母地位的改变而改变。[ ]
