举报
qqmxs2000
a1=22, S17=S9 (2a1+16d)*17/2=(2a1+8d)*9/2 (a1+8d)*17=(a1+4d)*9 17a1+136d=9a1+36d 100d=-8a1 d=-2a1/25 =-2*22/25 =-44/25 an=a1+(n-1)d =22+(n-1)*(-44/25) =-44n/25+506/25>0 44n/25<506/25 44n<506 n<11.5 即n=11时Sn的值最大 S11=(2a1+10d)*11/2 =[2*22+10*(-44/25)]*11/2 =[44-88/5]*11/2 =[22-44/5]*11 =[22-44/5]*11 =145.2