In this mysterious, math formula-filled world, a girl falls from the sky, and yes, she's Hezhi. Because her parents passed on her magic to the next generation, she had a talent that no one else could have, and she was born with the ability to do math. At the age of six, she can do all kinds of math problems, algebra, equations, geometry, functions, fractions...

Hezhi also has a biological brother - Heli, who has been naughty since he was a child, and has also caused a lot of trouble for his parents and Hezhi. But he has a cheerful personality and loves to laugh, and Hezhi also likes his younger brother very much.

However, their happy life is always so short-lived. Growing up, when Hezhi was seven and Heli was five, everything changed on their way home from school that day. His parents died in a car accident and failed to be rescued in time in the hospital. This caused Hezhi's heart to suffer a serious blow, and Heli also became very guilty. That day, Hezhi was lying on the bed crying, Heli suddenly knocked on the door and said, "Sister, don't cry, you don't have... Do you have the magic of math? Sister, you have to be strong!"

Now, Hezhi is fifteen years old and Heli is thirteen years old, and they are still living a happy and happy life, and they can go to and from school happily.

June 10, 2018 at 5:00 Pudong International Airport Terminal 1

"Jingle bell!" Heli's phone rang, he immediately picked up the phone and looked at it, it was Hezhi calling, he thought, "Hezhi should still be on the plane at this time, why did he call?" He immediately got on the phone and put it to his ear. "Heli, I'..... I got off the plane, I just got my luggage, you come to pick me up at the door. "Huh? Sister, I remember that your flight didn't take off until half past three, why did it arrive at five o'clock?" Hezhi said: "Alas, it was said that it would take off at half past three, but there were few passengers on the plane today, and I was afraid that they would be in a hurry, so the captain said that it would take off at two o'clock." Oh well, I'll be right away!"

"Heli! Heli!" After a while, Hezhi hurriedly ran over with his luggage, a happy smile appeared on his face, it seems that Hezhi has studied very successfully this time, Heli said: "Sister! I knew you had a successful trip!" Truly! You guessed it! This time I went to Japan, where I learned about Japanese math skills. Hezhi said, "Let's go to Lu Xun Park tonight, and I'll treat you to something to eat." They walked and talked and walked into the subway.

6:30 p.m. Hongkou Football Stadium

"By the way, Heli, you're about to have your final exams, right?" Hezhi asked while eating, and Heli nodded. "Then you're almost up to the mark, right?" Hezhi asked. At this time, Heli shook his head helplessly and sighed: "Math is not good... Hezhi said, "..... Why don't you always fail in math?" At this time, Heli said: "Sister, you don't understand, the math teacher gave us ten tutoring newspapers, eight practice papers, the concept was written silently again, and we were given a mock test, but my math scores always couldn't go up, you see my Chinese, English and physics scores can be seen, but only mathematics, but the grades can't go up, what is the reason?"

"You take the paper for the mock exam."

When Heli took out a roll full of red forks and gave it to Hezhi, Hezhi looked at it again and again..."x²+6x+9, find the discriminant formula of the equation, this is actually wrong!" What a pity!" Sister, you have to think of a way, otherwise I won't be able to pass the high school exam in the future!" Hezhi said helplessly: "In this way, I will give you a magic pen."