First of all, this chapter does not actually require mastery. I don't know if you want to take part in the competition. In fact, the competition will not be tested. Here Y 1V 1'+Y2V2'=0 is purely for the convenience of calculation. It is in this book because y'' does not contain V 1' and V2'. In fact, you can also do V 1=V2 and so on, as long as it is an equation about these two quantities. Why?

Because you want to know two unknowns for two equations, now you have two unknowns, but you only need to satisfy one equation, so if you want to solve, you need an equation, you can set this equation. For example, if you want to solve the equation XY 1+XY2=0, (Y 1, Y2 is represented by x), obviously there are countless groups of solutions, if you want to solve one. The purpose of constant variation method is only one solution, so it will make Y 1V 1'+Y2V2'=0, and the purpose is achieved. This equation has also been explored by mathematicians, and it is found that this will simplify the calculation. If not, it may not be an equation.

Lala smashed a lot, just to let you know that this is really a bit difficult to understand, and teachers usually don't talk about it in class.