s4=4a1+4*3*d/2=4a1+6ds9=9a1+9*8*d/2=9a1+36ds4=s9 -->4a1+6d=9a1+36d5a1=-30da1=-6dsn=na1+n(n-1)d/2=-6nd+n(n-1)d/2=d/2(n^2-n-12n)=d/2((n-13/2)^2-169/4)所以,n=6,或n=7时,Sn取最大值