打开App
360P
小升初常考题型，等差数列求和公式记不住，那是没理解推导过程
和=（首项+末项）×项数÷2
小学简便计算4/13
App01:54
App01:44
App01:16
App01:51
App03:06
App01:54
App01:32
App02:01
App03:02
App01:26
App02:24
App01:18
App01:54打开App观看
和=（首项+末项）×项数÷2
App01:54App01:44App01:16App01:51App03:06App01:54App01:32App02:01App03:02App01:26App02:24App01:18App01:54
