等差数列{an}的首相a1>0,前n项和为Sn,若3a5=8a12

问题描述:

等差数列{an}的首相a1>0,前n项和为Sn,若3a5=8a12
则Sn最大时,n的取值是?

3(a1+4d)=8(a1+11d)=>a1=-15.2d=>a16=-15.2d+15d=-0.2d>0,(因为d