a1=S1=1-48=-47
n>=2:
an=Sn-S(n-1)=[n^2-48n]-[(n-1)^2-48(n-1)]
=n^2-48n-(n^2-2n+1-48n+48)
=2n-49
a1=2*1-49=-47,符合.
所以,通项是an=2n-49