Sn=-n^2+10n,
a(1)=9,d=-2,an单调减
a(5)=1,a(6)=-1> a(7) > … > a(n)
n>5时,
Sn'=|a(1)|+|a(2)|+…+|a(n)|
=a(1)+a(2)+a(3)+a(4)+a(5)-a(6)-a(7)-…-a(n)
=S(5)-(Sn-S(5))
去绝对值看的清楚些
Sn=-n^2+10n,
a(1)=9,d=-2,an单调减
a(5)=1,a(6)=-1> a(7) > … > a(n)
n>5时,
Sn'=|a(1)|+|a(2)|+…+|a(n)|
=a(1)+a(2)+a(3)+a(4)+a(5)-a(6)-a(7)-…-a(n)
=S(5)-(Sn-S(5))
去绝对值看的清楚些