@[frankfan7707](/user/615297) 等差数列套公式啊
by D_M_ @ 2022-04-16 19:18:41
@[frankfan7707](/user/615297) （首项+末项）*项数/2
by D_M_ @ 2022-04-16 19:19:33
AC了，感谢dalao
by frankfan7707 @ 2022-04-16 20:37:19