S3=S4-a4=21/2
a1+a2+a3=21/2
a1+a3=2a2
所以3a2=21/2
a2=7/2
则2d=a4-a2=-2
d=-1
a1=a2-d=9/2
所以an=-n+11/2
Sn=(a1+an)n/2
=(-n²+10n)/2
=[-(n-5)²+25]/2
所以n=5,Sn最大=25/2
S3=S4-a4=21/2
a1+a2+a3=21/2
a1+a3=2a2
所以3a2=21/2
a2=7/2
则2d=a4-a2=-2
d=-1
a1=a2-d=9/2
所以an=-n+11/2
Sn=(a1+an)n/2
=(-n²+10n)/2
=[-(n-5)²+25]/2
所以n=5,Sn最大=25/2