系统原理图中三楼的第五个耦合器编号应怎样编？十九楼的第一个功分器呢？一楼的第一个衰减器呢？
参考答案：
T5-3F；PS1-19F；AT1-1F。
若a∈[-1，1]，b∈[-1，1]，求关于x的方程x2+ax+b2=0有实根的概率．
He _____ in the classroom just now. he _____ be there now. [ ]
A. heard to sing, may
B. was heard sing, must
C. heard sing, must
D. was heard to sing, may
