R121-212 之R 322R 2 1 多32R 依此类推可推到n级。R R 3 R 2 3 R 1 2 2 0 1 3 1 2 2 1 1 2 2 2 1 1 2 2 2 1 1 2 2 2 V i R V i i R V i i R V i i R V i i R = = = = = = = = = 依此类推可推到n级