s = 0 +2*1 ,k = 1+1; s = 2,k = 2s = 2+ 2*2,k = 2+1;s = 6,k = 3s = 6+2*3,k = 3+1; s = 12,k= 4s = 12+2*4,k = 4+1; s = 20,k = 5s = 20+2*5,k = 5+1;s = 30,k = 6s = 30
30 纯口算😂
10
30
