设{an+3nd}数列各项为b1,b2,b3.........bnb1=a1+3d, b2=a2+6d, a3=a3+9d. ........ ........bn=an+3nda(n+1)>an对吧，而且3（n+1）d>3nd,所以bn>b(n-1)>.................b3>b2>b1{an+3nd}递增
