等差数列{an}中,a1>0,3a4=7a7,sn是数列{an}的前n项和,则sn取得最大值是n=
问题描述:
等差数列{an}中,a1>0,3a4=7a7,sn是数列{an}的前n项和,则sn取得最大值是n=
答
3a4=7a7;3(a1+3d)=7(a1+6d);所以-33d=4a1;d=-4a1/33;
当n=9是,a9=-3a1/33