已知工频正弦交流电流在t=0时的瞬时值等于0.5A，计时始该电流初相为30°，求这一正弦交流电流的有效值。
参考答案：
0.707A
We would rather die than ______ difficulty. [ ]
A. give up to
B. give in to
C. give way to
D. give back to
延续注册，注册有效期为（）年。
A.2
B.3
C.4
D.5
