an=sn-sn-1=2n-5bn=1-5/2n Tn=b1+b2+b3+.......+bn=n-5/2(1+2+3+....+n)=n-5/2(n ^2+1/2) =-(5n^ 2+n/4)
n=1时，a1=s1=1n>=2时 an=sn-s(n-1）=2n-5
