The general term formula of known sequence {an} is an = n ^ 2-21n + 20? Answer: if the sum of the first n terms of a sequence is the smallest, then there is an

The general term formula of known sequence {an} is an = n ^ 2-21n + 20? Answer: if the sum of the first n terms of a sequence is the smallest, then there is an

The general term an = n & # 178; - 21n + 20 is a quadratic function whose domain of definition is n *, which indicates that each term of {an} has positive, negative, and zero. Because when we find out the value of N, Sn is the smallest, so we only need to find out several terms of an ≤ 0 A19 is negative, while A21 is positive, so S19 = S20

If a1 + A2 + a3 + A4 + A5 = 31, A2 + a3 + A4 + A5 + A6 = 62, then the general formula of this sequence is

（a1+a1q+a1q^2+a1q^3+a1q^4）÷（a1q+a1q^2+a1q^3+a1q^4+a1q^5)=1/2

How to find the general formula of sequence 1,4,6,11,15?
The respondent knelt down to thank him,
I'll give you at least five points,

An = n (n + 1) / 2 (when n is odd)
An = n (n + 1) / 2 + 1 (when n is even)
The synthesis is: an = n (n + 1) / 2 + [1 ^ n + (- 1) ^ n] / 2

Sequence 1.3.6.10.15 What is the recurrence formula for?
A. An + 1 = an + N, n belongs to positive integer B. an = an-1 + N, n belongs to positive integer and N is greater than or equal to 2
Why choose B instead of a?

A2 = a1 + 2, so a is wrong. An + 1 = an + N + 1, n is a positive integer
And N in B starts from 2, so it's right