Coming out of the secret room, the east has been white, and the morning star illuminates the first rays of summer, so that the sleeping Qiantang City slowly opens its eyes, and the burdens, hawkers, gatherers, and workers converge from all directions, like flowing blood, making the whole city instantly vivid.

Xu You yawned and returned to his residence, Qingming went to knock on the door first, Yu Yu opened the door with sleepy eyes, and saw Xu You's figure standing in the courtyard, his blue eyes overflowed with surprise that could not be concealed, he ran forward two steps suddenly, and hurriedly stopped, his hands tightly pinched the corners of his clothes, and he saluted him in a restrained manner.

Nanren's etiquette, she has learned better than the real Nanren!

Since Yu Yu was sent to serve Xu You, Xu You began to disappear for three days, and then directly retreated without seeing anyone, and the time they spent together was counted on one hand, in fact, they were not as familiar with each other as the usual master and maid.

"Where's the ugly slave?"

The ugly slave is eleven years old, and he has the appearance of a little girl, and now it is not convenient to live in the same room with Yu Su, anyway, there are three main rooms and four partial rooms in this courtyard, and a few more people are enough to live.

"I'm asleep, she doesn't know that Lord Xun is back, or should I wake her up?"

"No need!" Xu You looked at Yu Su, she obviously just got up from the bed, her clothes were disheveled, her hair was scattered at random, the body of a mature woman always inadvertently exuded a charming smell, if it weren't for the scars on her face were really terrifying, I was afraid that it would make any man crazy, and said with a smile: "You can also sleep, I have Qingming here, nothing is wrong." ”

Entering the inner room, of course Yu Su would not go to sleep like this, got dressed, cleaned his hands, prepared a change of clothes for Xu You, went to the side room to warm the water for the small stove, and then carried the stool, sat at the door in a regular manner, waiting for Xu You's orders.

A red sun was rising, and the hot weather made the cicadas in the forest shout at the top of their throats, but Yu Su's heart was very calm, and there was a clear difference from the wandering and restlessness when he couldn't see Xu You before-this Nanren Lord was not very old, but there always seemed to be a kind of magic power in his body, which could give his followers indescribable confidence and reliance.

The sun is like fire, pouring out wantonly,

It's another muggy day!

"Ahh

Xu You turned over and sat up, beads of sweat oozed slightly on his head, he had a dream just now, dreaming that all the statues of the gods and lords who had been destroyed by the flood came to life, one by one, more than a thousand feet high, looking down around him, his eyes scattered with golden light were as cold as snow, and the boundless horror and horror suddenly penetrated into the depths of his soul, making people tremble.

Qingming came in when he heard the sound and said, "Lang Jun? ”

Xu You waved his hand, slowed down, raised his head and smiled: "I had a nightmare...... A bunch of dead ghosts pretending to be fairies scare me! Ha, it doesn't matter! ”

Qingming was stunned and didn't understand what Xu You meant, but he didn't ask, but cautiously checked the surroundings to make sure that there was nothing abnormal before letting Yu Yu in.

Just cleaned up under Yu Su's service, and pushed the door in the winter solstice. She was still in the city to deal with things when Xu You returned to the mountain last night, and she couldn't rush back because of the curfew, so she was almost the first to gallop out of the city early this morning, lest it be too late, and Xu You would be gone.

"Xiaolang!"

Dong Solstice rushed over directly and hugged Xu You, her emotions were unrestrained and exposed, her eyes were already red, and she sobbed: "You're finally back!" ”

Xu You touched her head and said softly: "Yes, I will come back safely, and I don't have to worry about it in the future!" ”

"Hmm!" Dong Solstice nodded vigorously, reluctantly left Xu You's warm embrace, wiped the teardrops from the corners of his eyes, and said with a smile: "I know that nothing in the world can stump Xiaolang!" ”

Xu You is not as optimistic as her, this time he sneaked into Heming Mountain, it can be described as nine deaths, a little careless, the little life has to be explained there, and said: "It's just a fluke!" ”

In front of Yu Yu, these inside information could not be revealed, and after a few simple words, Xu You asked Dong Solstice to gather everyone in the square halfway up the mountain. It is said to be a retreat to the outside world, but after such a long time, it is time to show up, so as not to make people's hearts unstable and live in vain!

Seeing Xu You successfully get out of the customs, those tenants were all filled with sincere joy, after all, only Xu You was there, Mingyu Mountain was there, and with Mingyu Mountain, they had a place to settle down, so that they would not be displaced, and they didn't know tomorrow today.

A few words of comfort, learned that this year's harvest is excellent, in addition to grain, tea gardens and orchards are also harvested, Xu You specially found this season's pre-Ming and pre-rain tea, the buds and leaves are delicate, the color is green and fragrant, but now people are violent and ruined, only know how to chew peonies, swallow hard, and completely waste these good teas.

After finishing his trivial work, he went to see Zu Hu again. When he left Qiantang last year, Xu You secretly made many arrangements, one of which was to give Zu Xiao a book called Tianjing Yusuan, which recorded in detail Ali Lab numbers and addition, subtraction, multiplication and division, as well as the specific solutions of elementary equations, the calculation of the perimeter area and volume of various shapes, fractions, decimal algebras, and so on. These things are very simple, and the topics in the "Nine Chapters of Arithmetic" are not as complicated, but the so-called Tianjing jade calculation is a kind of mathematical operation rules that are completely different from the arithmetic methods of this era. For example, she is only rough in arithmetic, so she learns twice the result with half the effort and makes rapid progress. But Zu Xiao is different, he has stood at the peak of the mathematical world of this era, many conventional rules have almost become his instinct, to change his way of thinking, and then climb step by step from the foot of the mountain, it is indeed a little difficult.

However, once he mastered this kind of mathematical thinking, Zu Xiao would be the one in the Northern and Southern Dynasties who was most likely to fully master the Heavenly Scripture and Jade Calculation, except for Xu You, and could even integrate the two and open the door to a new world.

Everything counts - Pythagoras!

Zu Hao's room was not much different from what Xu You had seen in the barracks, the same mess and no place to put his feet. However, there is also a slight difference, what used to be a huge number of counting chips in the room is now a room full of papers. Xu You picked up one casually, and there were various equations densely written on it, from one yuan to two yuan once, and the answers were very detailed. It turned out that Zu Xiao had learned and used the equation solution method given to him by Xu You to re-solve the previous famous arithmetic problems, which could not only verify the right and wrong, but also deepen his understanding.

"Sir!"

"Sir!"

Zu Yu was solving the problem at the desk, Xu You shouted several times before he woke up from his addiction, looked at Xu You in a trance, and was stunned for a moment before hurriedly standing up. He has always been unconventional, grabbed Xu You's hand, and said, "Come and see, how to solve this problem?" ”

Xu You followed him to the front of the case, only glanced at it, and almost dropped his jaw, this turned out to be a cubic equation problem. He touched his nose and said with a wry smile: "Sir, I only let you learn elementary jade arithmetic, why did you start solving cubic equations?" ”

"At the beginning of April, there was a sudden heavy rainfall, and the Cuiyu Lake on the west side of Mingyu Mountain swelled, in order to prevent flooding of the fields, it was necessary to build an embankment, but in view of the topography, the east and west ends of the embankment were different in width and narrowness, and the height was also different, how to ensure the continuation of the project under the premise of determining the manpower and the number of days, and the problem of the cubic equation was generated......

Xu You knew that it was Wang Xiaotong of the Northern Zhou Dynasty who first proposed the cubic equation and gave the solution, and the reason was the discovery made during the study of civil engineering. However, later after Qian Baocong's research, Zu Chongzhi could solve cubic equations in the Northern and Southern Dynasties, and he was more clever than Wang Xiaotong. Zu Yu and Zu Chongzhi in front of him are not the same person, but their trajectories are becoming more and more similar, many times, Xu You will lose his sense of awe and recognition of time and space in this plausible illusion, as if he is still in that familiar world, but he has accidentally crossed a thousand years!

"To put it simply, to solve cubic equations, we should follow the principle of 'quotient is always positive, reality is always negative, from constant is positive, and benefit is always negative'......"

Xu You told Zu Xiao the basic method of Qin Jiushao's algorithm, and then watched him go from thoughtful to ecstatic, and even Xu You didn't bother to say hello and wanted to return to the wonderful realm of solving the problem.

"Don't worry, wait first!" Xu You couldn't help it, he couldn't be too polite to this number idiot, so he hurriedly stretched out his hand to block his way, and said, "Mr. help me another favor, I'll teach you the factor method and the commutation method to solve cubic equations!" ”

Zu Hao's eyes lit up and said, "There are so many solutions?" ”

Xu You nodded affirmatively and said, "In addition to these, there is also a heavenly scripture formula that can solve all cubic equations." He called all theorems and equations the Celestial Formula, so that it was easy to understand and unify.

"Good!"

Zu Yu finally put down the brush in his hand, and went out with Xu You to go down the mountain to the county seat. There was an ox cart on the road, and Xu You asked in the bumps: "Sir, the ox cart only has two wheels, and it sways back and forth, and the ride is neither comfortable nor fast, so why not use four wheels?" ”

"Four-wheeled vehicles have been around since the Qin Dynasty, and although they are more comfortable than two-wheeled vehicles, they consume animal power and are worth a lot of money; Second, it is difficult to turn and inconvenient to enter and exit, so it gradually declines. As for now, Lang Jun, please look at the ...... on the ground"

Xu You looked down and saw that due to the heavy rain before, the ground was muddy and piled up, and it was crushed by various ox carts and firewood wheels just after it was sunny, forming thick rut marks. The wheels of the ox cart they rode in were like the fixed tracks of a train, and it was almost impossible to get out of the tracks.

Zu Xiao smiled: "For so many years, everyone has used two-wheeled vehicles to travel, due to the unification of the size of the axle, the road has formed this deep and solid rut marks, if you use a four-wheeled vehicle, it is easy to say when you are straight, what to do when you detour?" The steering angle of a four-wheeler is much greater than that of a two-wheeler, and it is impossible to use rut marks at all......"

Xu You thought for a while and said, "Sir, if you only build four-wheeled vehicles for dignitaries, which one do you think is the most difficult to solve among these problems?" ”

"The hardest thing is turning!" Zu Xiao said without thinking: "Even if the four-wheeled vehicle is built, ordinary people can't afford it, so animal power is not a problem for the gate lord clan." As for the rut prints, in fact, it doesn't matter, nobles generally do not come to these country roads, if in the city, or take the post road, it is the avenue such as a stone, its straight as an arrow, as long as the steering problem is solved, the four-wheeled vehicle runs fast and steady, naturally for the best choice! ”

There is a verse in "The Book of Songs: Xiaoya" that "Zhou Dao is like a stone, and it is as straight as an arrow", which means that the ancients already knew how to build roads with rammed earth and use lime to stabilize the soil, and the roads made were as hard as grinding stones and as straight as lines. In the Qin Dynasty, it was a miracle to build Chi Road, Straight Road, Wuchi Road and New Road with Xianyang as the center, and build a road network all over the country.

At that time, the reason why ancient China did not transition from two wheels to four wheels as in Europe was that the lack of steering devices was the biggest obstacle in addition to the terrain and road problems. Xu You knows this very well, so since He Wei said that the use of money is tricky, he has put the idea of making money on the four-wheeled ox cart, which does not need to be built more, let alone popularized, as long as those gate valve aristocrats realize that riding a four-wheeled vehicle is a status symbol, and it is indeed more comfortable and faster than a two-wheeled vehicle, sales and income are no longer a problem!

"Turn," Xu You smiled slightly, "Actually, it's not difficult!" ”

Zu Xiao was stunned!