设公差为d.(1)a6-a3=3d=21-3=18d=6a1=a3-2d=3-12=-9an=a1+(n-1)d=-9+6(n-1)=6n-15数列{an}的通项公式为an=6n-15.(2)Sk≥55ka1+k(k-1)d/2≥55-9k+ 6k(k-1)/2≥553k²-12k≥553(k-2)²≥67(k-2)²≥67/3k...