3a8=5a13
3(a1+7d)=5(a1+12d)
a1=-39d/2
an=a1+(n-1)d=-39d/2+(n-1)d≤0
(n-1)d≤39d/2
n≤39/2+1=20.5
a20>0
a21<0
n=20
s20最大