Chapter 28

"Qin Luo, you get up and talk about the process of solving this problem." At this time, Li Wei's "amiable" words brought Qin Luo back to reality.

Let Sn be the first n terms of the arithmetic sequence {an} and always A1 = - 7s3 = - 15

(1) find the general term formula of {an}.

(2) Find Sn and find the minimum value of Sn.

This is a minimum problem of arithmetic sequence.

The first one is relatively simple, and only the general term formula can be obtained, but the difficulty of the second question increases sharply.

Not only the value is required, but also its minimum value is found.

Mr. Li, this is to create opportunities for me to pretend to be forced.

It's a rare teacher in a hundred years.

At this moment, Li Wei was no longer a black faced evil spirit in Qin Luo's eyes, but a small white flower without sorrow and injury.

Qin Luo went to the blackboard and picked up the chalk. He did not answer immediately. He was lost in thought.

What kind of method should you use to pretend to be forced?

Five minutes later, Qin Luo didn't write a word. He stood in front of the blackboard in a daze.

"Qin Luo!" Li Wei's face was livid and yelled, "what are you waiting for?"

"Emmm, I'm trying to solve it that way."

"Why, you still have several ways?" Li Wei came to be interested. For the arithmetic sequence, the knowledge of high school covers a solution. He is very curious about what kind of solution Qin Luo can "create".

"Not much, not much, just three."

“……”

"The first." Qin Luo raised his hand to write a capital one on the blackboard, and then quickly began to solve the problem.

The general formula of {an} is an = 2n-9

(2) by SN = n? - 8N = (n-4) so when n = 4, Sn is the minimum value, the minimum value is - 16

from A1 = - 7, D = 2

so the general term formula of {an} is an = 2n-9

(2) Sn = n? - 8N = (n-4) 2, so when n = 4, Sn is the minimum value, the minimum value is - 16

the second kind

does not stop writing.

“f＇(ξ)*(b-a)=f(b)-f(a)…… The minimum value is - 16 "

" the third formula is an = 2n-9... " 

one, two, three!

Three methods, the results are the same!

But each method is very different.

Quiet, dead quiet.

Of the 47 students in the classroom, none of them spoke.

It's not that they don't want to say it, but they don't say it!

Qin Luo is such a bully that he can even use the contents of the University. Can they talk about it?

You know, knowing is one time, flexible use is another thing.

Qin Luo can use the University method to solve problems, which proves that he has already mastered the method.

"In the last way, you used split term elimination?" After a long silence, Li Wei broke the silence in the classroom.

"Yes." Qin Luo nodded.

"I didn't learn that."

"Self taught."

"Where did you learn?" Li Weixiang asked about the content of the elective course in high school.

"Lagrange's mean value theorem and Ptolemy's theorem"

Li Wei was shocked: "are you learning high numbers?"

"Well, I've been studying Goldbach's conjecture recently."

500 points of learning Ba integral, Qin Luo is salivating at him.

“……” One second, Li Wei was still elated, but the next second he was as speechless as a fly.

Goldbach conjectures that a high school student can study it. Do you think you are Tao Zheyuan or Qiu Chengtong?