二甲苯的指标是多少？
参考答案：二甲苯的指标是≥98.0％。
—Are you free tomorrow? I was wondering if we could have lunch together.
— No, I'm afraid not. I John then.
A．am meeting
B．met
C．have met
D．was meeting
设f(x)是周期为2π的周期函数，在一个周期[-π，π]上的表达式为，试写出f(x)的傅里叶级数的和函数在x=-π处的值．
