A:spent; must have bee
B:cost; ought to have spent
C:wasted; wouldn’t have bee
D:took; can’t have spent
一个数加上它的50%等于7.5,求这个数.
设四面体的六条棱的长分别为1,1,1,1,2和a,且长为a的棱与长为2的棱异面,则a的取值范围是( )