已知数列an前n项和Sn=n^2-9n,第k项满足5<ak<8,则k等于多少

问题描述:

已知数列an前n项和Sn=n^2-9n,第k项满足5<ak<8,则k等于多少

Sk=k^2-9k S(k-1)=(k-1)^2-9(k-1)=k^2-11k+10 ak=Sk-S(k-1)=k^2-9k-(k^2-11k+10) =2k-10 所以5