|3-k|+|3-2k|=|4-k|+|4-2k|
1.k≤3/2時,a3≠a4;3/2＜k≤2時,k=2,a3=a4=2;2＜k≤3時,a3≠a4;3＜k≤4時,k=3,a3=a4=3;k＞4時,a3≠a4
2.經(jīng)驗證n≤5時,都成立
3.n≥6時,an=2n-6＞a3或an=2n-9≥a3
k=2或3