The method of addition, subtraction and elimination will not be analyzed here. Here, the simplest equivalent form of simultaneous equations is solved by addition, subtraction and elimination. The principle of addition, subtraction and elimination method is to directly apply a property of the equation, that is, the equation is still valid if the two sides of the equation add and subtract the same amount. Just a new type of child, only one yuan, thus simplifying. Substitution method. ①x+y=3 is equivalent to X = 3-Y, and the equivalent transformation here is simply that the two formulas before and after the transformation can be derived from each other. Because of this, we think that the two formulas before and after the transformation are essentially the same formula, not two different formulas, and they can all be represented by the same symbol ①. This is very similar to the relationship between equilateral triangles and equilateral triangles.