From the secret room, the east was already white, and the stars illuminated the first ray of summer, making the sleeping Qiantang City slowly open their eyes, carrying loads, hawking, market, and working, gathered from all directions, as if the blood flowed, which made the entire city lively instantly.

Xu You returned to his residence while yawning. Qingming went to knock on the door first. Yu Tu opened the door sleepily and saw Xu You standing in the yard. Unpredictable surprises were overflowing in his blue eyes. He ran forward two steps forward, then hurriedly stopped, pinching the corners of his clothes tightly with both hands, and saluting him restrainedly.

She has learned the etiquette of the Southern people even better than the real Southern people!

Since Yu Tu was sent to serve Xu You, Xu You began to disappear from the end of the year for several days, and later he went into seclusion without seeing anyone. They counted the time they spent together with each other, but in fact they were not as familiar with each other as usual.

"Where is the ugly slave?"

The Chou Nu is already eleven years old and looks like a little girl. It is not convenient to live in the same room with Yu Tu. Anyway, there are three main rooms and four side rooms in the yard, so a few more people are enough to live in.

"I went to bed, and she didn't know that the Lord was back, why should I wake her up?"

"No need!" Xu You looked at Yu Tu. She had obviously just gotten up from the bed, dressed in disheveled, and her hair was casually dispersed. The mature woman's body always inadvertently exuded a charming smell. If the scars on her face were not really scary, she would have made any man crazy. She smiled and said, "You go to sleep too. I have Qingming here, so it's nothing."

After entering the inner room, Yu Tu certainly would not go to sleep like this. He put on his clothes, cleaned his hands, prepared the clothes for Xu You to change, went to the side room to warm the small stove, then carried Hu benches, and sat at the door in a regular manner, waiting for Xu You's instructions.

A red sun was rising, and the hot weather made the cicadas in the forest shout at the top of their throats. Yu Tu's heart was very calm, which was obviously different from the floating uneasy when he could not see Xu You before. This southerner, Huangzhu, was not old, but he always seemed to have a magic power that could give his followers indescribable confidence and support.

The sunlight is like fire, pouring wildly,

Today is another hot day!

"ah!"

Xu You turned over and sat up, sweat beads oozing from his head. He had just had a dream, dreaming that all the statues of the God Lord who were destroyed by the flood came alive, one by one, more than a thousand feet tall, and looked around him. His eyes were scattered with golden light were cold as snow. The boundless horror and fear suddenly penetrated into the depths of his soul, making people tremble.

When Qingming heard the sound, he came in and said, "Mr. Lang?"

Xu You waved his hand, slowed down, raised his head and smiled, "I had a nightmare... A group of dead ghosts pretending to be gods to scare me! Ha, it's okay!"

Qingming was stunned for a moment, not understanding Xu You's meaning, but he did not ask questions, but carefully checked the surroundings and confirmed that there was no abnormality before letting Yu Tu come in.

Just after Yutu's service, she was cleaning up and pushed the door open and entered. Last night when Xu You returned to the mountain, she was still dealing with things in the city. Due to the curfew, she was almost the first to leave the city and galloping early this morning, for fear that Xu You would disappear again after this moment.

"Xiao Lang!"

The Winter Solstice rushed over and hugged Xu You. Her emotions were unrestrained and exposed, her eyes were already red, and she sobbed, "You are finally back!"

Xu You touched her head and said softly: "Yes, I'm back safely, so don't worry anymore!"

"Yeah!" Dong Xie nodded vigorously, reluctantly left Xu You's warm arms, wiped the tears from the corners of her eyes, Zhan Yan smiled and said, "I knew that nothing in the world could stump Xiao Lang!"

Xu You was not as optimistic as her. This time he sneaked into Heming Mountain, which was a life-and-death death. If he was not careful, his life would have to be explained there and said, "It's just a lucky person!"

In front of Yu Tu, these internal stories cannot be revealed. After a few words, Xu You asked the Winter Solstice to gather everyone in the square halfway up the mountain. He said that it was a retreat, but after so long, it was time to show up to avoid unstable people's hearts and quarrels!

Seeing Xu You leaving the seclusion successfully, the tenants were filled with sincere joy. After all, only Xu You was there and Mingyu Mountain was there. With Mingyu Mountain, they had a place to settle down and not be displaced. Today is not known tomorrow.

After a few words of comfort, I learned that the harvest this year was excellent. In addition to grain, the tea garden and orchards also had good harvests. Xu You specially found this season's Ming Dynasty, the tea before the rain, with delicate buds and leaves, and the color was green and fragrant. However, people nowadays were wasting natural resources and only knew how to chew peonies, swallow them raw, and swallow them hard, which completely wasted these good tea leaves.

After finishing the trivial matters, I went to see Zu Qi again. When I left Qiantang last year, Xu You made many arrangements secretly. One of them was to hand over Zu Qi a book called Tian Jing Yu Absorption, which records in detail the specific solutions to the Arabic numerals, addition, subtraction, multiplication and division, as well as the specific solutions to the primary equations, calculation of the perimeters, area and volume of various shapes, fractional algebra, etc. These things are very simple, and are not as complicated as the title in "Nine Chapters of Arithmetic", but the towers rise from the flat ground, so-called The natural classical calculation is a mathematical calculation rule that is completely different from the calculation method of this era. If you want to learn it, it is difficult to say it or easy to say it. For example, Lu Shuang, she is only a rough understanding of calculation method, so she can learn twice the result with half the effort and progress quickly. But Zu Yan is different. He is already at the peak of the mathematics world of this era. Many conventional rules have almost become his instinct. It is indeed a little difficult to change his thinking and then climb step by step from the foot of the mountain.

However, once you master this mathematical thinking, Zu Qi will be the one who is most likely to fully master the Jade Calculation of the Natural Classics in the North and South dynasties, except Xu You, and you can even integrate the two and open the door to a new world.

Everything is counted - Pythagoras!

Zu Yan's room was not much different from what Xu You saw in the barracks. It was also messy and had no place to put it in. However, it was also a bit different. The ones placed in the room were a large number of calculation sheets, but now there are papers floating around in the room. Xu You picked up a piece of it casually, and it was filled with various equations written on it, from one yuan to two yuan, and the answers were very detailed. It turned out that Zu Yan learned and used it to use the equation solution given to him by Xu You to resolving the famous calculation questions before, which can not only verify the right or wrong, but also deepen understanding.

"gentlemen!"

"gentlemen!"

Zu Yan was at the desk to solve the problem. Xu You shouted several times before he woke up from his indulgence. He looked at Xu You in a daze and was stunned for a moment before he hurriedly stood up. He had always been unruly and grabbed Xu You's hand and said, "Let's see, how should this question be solved?"

Xu You followed him to the front desk, took a look, and almost lost his chin. This was actually a cubic equation problem. He touched his nose and smiled bitterly: "Sir, I only let you learn the primary jade calculation. Why have you started to solve the cubic equation?"

"In early April, heavy rain suddenly fell, and Cuiyu Lake on the west side of Mingyu Mountain surged. In order to prevent flooding the fields, it is necessary to build a dike. However, given the terrain, the width and narrowness of the east and west ends of the dam are different, and the height is also different. How can we ensure that the project continues while determining the manpower and the number of days, the problem of cubic equations arises..."

Xu You knew that the first person to propose the cubic equation and give the solution was Wang Xiaotong of the Northern Zhou Dynasty. The reason was his discovery when studying civil engineering. However, after Qian Baocong's research, Zu Chongzhi could solve the cubic equation during the Southern and Northern Dynasties, which was even more sophisticated than Wang Xiaotong. Zu Zhu and Zu Chongzhi were not the same person, but their trajectories were becoming more and more similar. Many times, Xu You would lose his sense of awe and recognition of time and space in this illusion of right and wrong, as if he was still in that familiar world, but he had accidentally traveled forward for a thousand years!

"Simply put, to solve the cubic equation, we must follow the principle of 'Quantity is always positive, real is always negative, always comes from positive, benefit is always negative'..."

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

"Don't be anxious, wait first!" Xu You had no choice but to be too polite to this nude silly, so he hurriedly reached out to block his way and said, "Sir, do me another favor. I will teach you the method of factoring and the method of exchange of elements to solve the cubic equation!"

Zu Yan's eyes were shining and he said, "There are so many solutions?"

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

"good!"

Zu Qi finally put down the brush in his hand and went out with Xu You to go down the mountain to the county town. On the way, Xu You asked, "Sir, the ox cart has only two wheels, which are swaying forward and backward. It is not comfortable to ride or go fast, so why not use four wheels?"

"The four-wheeler was already in the Qin Dynasty. Although it is more comfortable than the two-wheeler, it consumes animal power and is valuable; the other is that it is difficult to turn and inconvenient to enter and exit, so it gradually declines. As for now, please look at the ground..."

Xu You looked down and saw that due to the heavy rain before, the mud on the ground was piled up, and after it cleared, it was crushed by various ox carts and trucks, forming thick rut marks. The wheels of the ox cart they were riding were walking in these two rut marks, just like a fixed track of a train, it was almost impossible to deviate from the track.

Zu Yan smiled and said, "For so many years, everyone has traveled with two-wheelers. Because the axle size is unified, the roads have formed such deep and strong rut marks. If you use four-wheelers again, it is okay to say when you are straight, but what should you do when you are detoured? The steering angle of the four-wheelers is much larger than that of two-wheelers, and you cannot use rut marks at all..."

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

"The most difficult thing is steering!" Zu Zhi said without hesitation: "Even if the four-wheeled vehicle is built, ordinary people cannot afford it, so animal power is not a problem for the aristocratic families. As for the ruts, it doesn't matter. The nobles generally don't come to these rural paths. If they are in the city or walk on the post road, the road is like a big boom, and it is straight as a arrow. As long as the steering problem is solved, the four-wheeled vehicle will run fast and steadily, so it is naturally the best choice!"

There is a poem in "The Book of Songs? Xiaoya" that says "The way is as bright as a thorn, and its straight as a arrow", which means that the ancients had already understood the method of ramming the earth and using lime to stabilize the soil. The roads created were as hard as grinding stone and as straight as lines. In the Qin Dynasty, Xianyang was centered on creating roads, straight roads, five-foot roads and new roads, building a road network all over the country, which was a miracle.

At that time, the reason why ancient China did not go from two-wheelers to four-wheelers like Europe was that in addition to terrain and road problems, the lack of steering equipment was the biggest obstacle. Xu You knew this well, so since He Ru said that money was difficult to use, he had put his idea of ​​making money on the four-wheeled ox cart. It didn’t need to be built more, and it didn’t need to be popularized. As long as those nobles realized that riding in four-wheeled ox carts was a symbol of identity, and it was indeed more comfortable and faster than two-wheeled ox carts, sales and revenue were no longer a problem!

"Turn," Xu You smiled slightly, "It's not difficult!"

Zu Zhi was stunned!
Chapter completed!