设y=x2•ex，求y’．
参考答案：
解析：本题考查的知识点是函数乘积的导数计算．
可以利用声音来传递__________和___________。
—How quiet it is here!
—Yes. We can hear nothing but the _____ of the running water.[ ]
A. music
B. sound
C. voice
D. noise
