A:none of them were here
B:no one was here
C:all of them were here
D:all of them were not here
设 a,b,c 为不全相等的正数,且 abc=1 ,求证:1a+1b+1c>a+b+c .
将一盏电灯放在桌子上代表太阳,在电灯旁放置一个地球仪代表地球,拨动地球仪模拟地球运动.读图完成下题.