a(n+1)=an-3
a(n+1)-an=-3,为定值.
a1=60,数列{an}是以60为首项,-3为公差的等差数列.
an=60-3(n-1)=63-3n
令an≥0
63-3n≥0
n≤21,即数列前20项均>0,a21=0,以后各项均