Question

3,4小题,小弟不才解不出来,

Answer 1

S17=S9

(a1+a17)17/2=(a1+a9)9/2

(2a1+16d)*17=(2a1+8d)*9

d=-2a1/25=-2

an=a1-2(n-1)=-2n+27

Sn=(25-2n+27)n/2=-n²+26n=-(n-13)²+169

所以n=13,Sn最大=169