我国少数民族主要分布在（    ）

（阅读二）选择正确答案。
    Jack is a teacher of English. He is not young but he is not old. He has a round face and black hair. He is tall. There are fifty students in his class. They all like him. Now it's in the afternoon. Look some students are studying in the classroom. Jack is there too. Jack helps them to study English. He is a good teacher and he is a good friend of them all.

如图，已知二次函数y= -x²+bx+3的图象与x轴的一个交点为A(4,0)，与y轴交于点B.
（1）求此二次函数关系式和点B的坐标；
（2）在x轴的正半轴上是否存在点P，使得△PAB是以AB为底的等腰三角形？若存在，求出点P的坐标；若不存在，请说明理由.

图示法是我们学习历史的一种方法。下列4幅两宋与西夏、金并立的示意图中，哪一幅是错误的？（    ）

若函数f（x）=﹣x•e^x，则下列命题正确的是（   ）