Question

设等差数列【An】满足A3=5,A10=-9,求通项公式,前n的和Sn及最大的序号n的值

Answer 1

A1+2d=5

A1+9d=-9

得d=-2

A1=9

An=9-2(n-1)=10-2n

Sn=9n-2(n-1)*n/2

=9n-(n^2-n)

=10n-n^2