中国所处的半球为( )
— Tom, look at those two girls!
—Oh, they look_________ because they are twin sisters.
如图(a)所示电路中,当闭合开关后,两只电压表的指针偏转均如图(b)所示,则电阻R1和R2两端的电压分别为( )
已知直线x+y=a(a>0)与圆x2+y2=4交于A,B两点,且|OA→+OB→|=|OA→﹣OB→|(其中O为坐标原点),则实数a是____