1.在等比数列中{an}中 S2=7 S6=9 求S4?
2个回答

1 s4比s2= s6比s4

s4=正负根号63

2 have been built是现在完成时延续性动词 were built是被动语态

3 分子分母上下同乘以根号3

4 took place是指事件发生了,以事件为主语 hold就是主办的意思,以主办人为主语.你可以通过词性区分,当中只有一个名词,再通过意思区分,找出主语是谁就迎刃而解.

5 数字间有关系吗?