"Theorem 2, if the function z=f(x,y) has a first-order and second-order continuous partial derivatives in some neighborhood of the point (x0,y0), and ...... Therefore, it is not possible to determine whether (x0, y0) is an extreme point. ”

Whew~, the last word fell on the blackboard, Liu Feng rubbed his sour arm, and threw half of the chalk into the chalk box, the chalk words are really not written by people, it seems that the teacher is not so easy to be!

Now that the last course is over, Liu Feng is over, and then bows to the audience: "Today's class is over, thank you for listening, I am Liu Feng, thank you!"

"Bang bang bang!"

A round of applause rang out, and it lasted for a long time.

"By the way, I forgot to assign today's homework, and the students turned to the after-class exercises, which are 67 pages, the first question, the third question, the fourth and fifth questions, ......" Liu Feng seemed to have completely entered the state and immersed himself in the great profession of a teacher.

Since you have lectured and don't assign some after-class homework, are you worthy of your classmates?

Students: "......"

You're addicted to being a teacher!

"Bang Bang!"

The applause of the exterminator finally rang out at this time, and she walked up to the podium while clapping.

"Teacher Yan, look ......" Liu Feng reacted immediately, and hurriedly abdicated to make way for Xian.

"Very good, classmate Liu Feng, it seems that the teacher has lost his eyes!" Yan Shan looked complicated and nodded, after all, he was still very satisfied with Liu Feng's class, "The teacher has gained a lot from this class!

"Okay, okay, Teacher Yan, as long as you don't hang up on my subject!" Liu Feng nodded and said.

"What kind of subject to hang, as long as you listen well in class, abide by classroom discipline, and play to your level in the final exam, why should the teacher hang up your subject!" The exterminator waved his hand and drove Liu Feng out of the classroom, staring at the classroom full of students, and continued, "I read the after-class homework assigned by Liu Feng just now, but it is not enough, in addition to the questions just now, the 7th question, the 8th question, and the 11th question ......"

Students: "......"

Sure enough, the exterminator is too ruthless!

"Huh~" Liu Feng finally breathed a sigh of relief, it seems that the small money of 1,000 yuan has been kept again.

But if it continues like this, it's not a problem, after all, letting a large number who has mastered all the courses and reached the full level stay in the novice village all day is not only a waste of resources, but also causes discomfort on the screen, if it is blocked by someone who can't think of it......

Liu Feng was not happy to think about it.

But what can be done?

The power of life and death is all in the hands of others, in case one day he is named like today, he can't come up and show it every time, after all, in addition to high mathematics, there are other 9 professional compulsory courses, and he must not be exhausted to death when he is a teacher in class all day!

No, we can't go on like this!

Walking on the steps of the classroom, Liu Feng secretly made a decision in his heart......

"Fengzi, here and here!" classmate Shi Shang looked at the competitors around him who were competing for Liu Feng, the 'king of popularity', and finally took Liu Feng under his command proudly by virtue of the advantage of being tall and big......

"After class!Liu Feng, come with me!"

Liu Feng had just sat on the stool next to Shi Shang's ass when he heard the news of the extermination teacher's announcement of the end of class, and the two of them chatted privately, cough cough......

Shi Shang was just about to inflict the ten major Manchu tortures on Liu Feng in order to torture the inhuman performance of this thing just now, as well as the harm to Sun Xiaoxiao's classmate, but now he could only do it, waved his hand, this thing looked at Liu Feng like a strong man Jing Ke, waved at him and said: "Go, my warrior! The wind is sluggish and the water is cold, and the strong man is gone......

Just kidding, this is an exterminator!

"you, you big-headed ghost!" Liu Feng glanced at Shi Shang angrily, and threw the textbook in his hand to him, "The textbooks are collected by Lao Tzu, take them back to the dormitory, and I'll go back when I go." ”

"And!" Lao Zhou raised a big middle finger.

The other students who wanted to get to know Liu Feng after class were already ready to flock to him, and when they saw this, they had no choice but to give up.

However, when he walked to the corner of the classroom, Liu Feng turned around and saw that Sun Xiaoxiao's classmate had squeezed away the big man who was full of screens, Shi Shiran came to Shi Shang's side, as if he was inquiring about something, and Shi Shang's goods were full of pugs......

……

Outside the office, Liu Feng knocked lightly on the door.

"Please come in!" said the exterminator's soothing voice.

Pushing open the door, Liu Feng entered, and saw that she was packing up the materials on the table.

"Teacher Yan. He greeted politely, "I'm here to help you." ”

"No, I don't. Yan Shan politely refused, "Liu Feng, do you know what I mean by letting you come?"

"Well, I don't know. "Liu Feng is very honest.

"Pedal!" shook the information in his hand, put it aside, and only heard Yan Shan say, "Liu Feng, your score in the final exam last semester was 20 points, but today the teacher saw your performance on the podium, it shouldn't be like this at all, do you have anything to say?"

"Teacher Yan, in fact, the reason why I only scored 20 points in the exam last semester is because of ......" Liu Feng told the story of improving his family conditions by working part-time all the time.

In fact, to be honest, if his family conditions really allowed, how could he go out all day to work part-time, save for tuition, and earn living expenses?

Liu Feng's parents are authentic farmers, and they can only go out to work occasionally, and the family's main business is farming, and the income can only be said to support the family, and there is no extra spare money.

In addition, last year, my grandmother was paralyzed at home, and someone had to watch and serve all the time, and the family that was not rich was even worse, and owed a lot of debt, and Liu Feng's tuition fees for college were still funded by the villagers!

After listening to Liu Feng's statement, Yan Shan sighed, it seems that she really wronged him, this kid is really strong: "Doesn't the school have scholarships, plus student loans or something, it should be able to support you for four years of college!"

Yan Shan asked the doubts in her heart, after all, with the strength shown by Liu Feng, the national first-class scholarship or something, is it not easy to catch?

Listening to Yan Shan's question, Liu Feng smiled bitterly in his heart.

Student loans or something, to be honest, Liu Feng was just a stunned young man when he first went to college, he didn't understand anything, and the people in the village didn't have much education, let alone understand, plus he wasn't admitted to a university like Shuimu or Beijing University, and no one paid attention to them.

As for the scholarship, how can Liu Feng, who has just gone to school, have the confidence to get it, after all, although he is still a little famous in the town, and he can be regarded as a scholar of all sizes, but when he arrives in the capital, this place where the elites of all ethnic groups in Huaguo, Tiannan, Haibei and North gather, where did Liu Feng get the courage to get the scholarship!

As for the strength he shows now, hehe, he has no strength without plugging, he used to work hard, even if he worked hard and worked hard, he was at most above average in school.

To be honest, he is really not the kind of person who is talented, and with his middle-aged qualifications, being able to be admitted to Chemical University is already the result of hard work.

Regarding Yan Shan's question, Liu Feng is really difficult to answer, saying that all he has is performing now because he can control the particles of the microscopic world.

Then he's not a fool!

Therefore, Liu Feng could only be silent.

Seeing this, Yan Shan is not easy to get to the bottom of it, after all, who doesn't have a difficulty, who doesn't have a secret?

"So have you earned next year's tuition? Although the teacher is not a wealthy family, he can still get tens of thousands of yuan......

In this regard, Liu Feng was very moved, although everyone called Teacher Yan the extermination teacher, but from the bottom of her heart, the classmates respected her very much, after all, in the university, she is so responsible and kind, and the point is that she is so rich and charming...... ,Ahem,,There are too few teachers.,It's too few.。