an=13-4(n-1)=-4n+17 ,让an>0求的n<=4,这里分两类情况(1)当n<=4,tn=(a1+an)*n/2=15n-2n^2,(2)当n>4,tn=t4-(a5+a6+……an),易得t4=28,a5+a6+……an可看成a5为首项的等差数列,a5+a6+……an=(-3+17-4n)*(n-4)/2=-2n^2+15n-28,所以tn=2n^2-15n+56,综上当n<=4,tn=15n-2n^2,当n>4,tn=2n^2-15n+56