来自刘美轮的问题
等差数列{an}中,a1>0,且3a1=5a13,则{sn}中最大项为?
等差数列{an}中,a1>0,且3a1=5a13,则{sn}中最大项为?
1回答
2020-04-05 21:44
等差数列{an}中,a1>0,且3a1=5a13,则{sn}中最大项为?
等差数列{an}中,a1>0,且3a1=5a13,则{sn}中最大项为?
解:根据题意:首项为a1,公差为d
3a8=5a13
因为:a8=a1+7d
a13=a1+12d
所以:
3(a1+7d)=5(a1+12d)
3a1+21d=5a1+60d
a1=-19.5d
即:a20=a1+19d>0
a21=a1+20d