One day, add, subtract, multiply and divide to go to the movies. After buying the tickets, they are going to walk into the cinema hand in hand. Suddenly, the conductor stopped them and said, "You can't go in at the same time, you have to go in one by one." After listening to addition, subtraction, multiplication and division, they began to quarrel and said go first. Finally, they decided to look for the wise old man to judge. When we arrived at the wise old man, we added, subtracted, multiplied and divided and asked, "Grandpa Wisdom, the four of us went to the movies. Who is in front? " "Which one of you has brackets?" The wise old man asked with a smile. "We have brackets." Add and subtract. "Then the addition and subtraction will go ahead." The clever old man replied. "Why does addition and subtraction precede multiplication and division?" Multiplication and division means unconvinced. "As long as who brought the brackets, who will go first. Multiply and divide without brackets, then add and subtract. " The clever old man explained patiently. Add, subtract, multiply and divide. After thanking the wise old man, he was about to leave when the problem came again. Addition and subtraction have brackets, so who comes first? Who ranks second? There are no brackets in multiplication and division, so who ranks third? Who ranks fourth? Add, subtract, multiply and divide, and run back to ask the wise old man. The wise old man said: "addition and subtraction are equal sisters, and whoever walks in front will go first;" Multiplication and division are also equal sisters, and whoever walks in front will go first. "After listening to the words of the wise old man, I fully understood the addition, subtraction, multiplication and division before I went to the movies happily.

As early as 3600 years ago, the mathematical problems written on papyrus by ancient Egyptians involved equations, that is, equations containing unknowns. Around 825 A.D., a mathematician from Central Asia, Alva Razimi, wrote a book called "Elimination and Return", focusing on solving equations. The Chinese word "equation" comes from the ancient mathematical monograph "Nine Chapters of Arithmetic", the eighth volume of which is called "equation".

An equation refers to an equation containing unknowns. It is an equation that represents the equal relationship between two mathematical expressions, and the value of the unknown quantity that makes the equation established is called "solution" or "root". The process of finding the solution of the equation is called "solving the equation". By solving the equation, we can avoid the difficulty of reverse thinking and directly list the equations with the quantity to be solved.