In mathematical problems, v often represents speed, which should be to find an unknown number, but here v replaces the commonly used X. 1 is actually used to distinguish the unknowns that are also represented by V, just like you have to learn to find two roots of an equation in the future. It has no practical significance to represent the two roots of an equation with X 1 and X2, just to distinguish. After 2 stands for square, you will understand a lot. .