The algebraic expression contains an unknown number. Let's give an example: for example, (x-2).

In the property (1), it is an algebraic expression, so it is an (x-2). At this time, no matter what value X takes, it is true.

But if the property ② is: "Both sides of the equation are multiplied or divided by the same algebraic expression", then it is also an algebraic expression of (x-2).

If we divide by, there may be a solution of x=2 in the result, and this algebraic expression will get 0 when we find the modern number (x-2). Then it is meaningless for us to divide by this algebraic expression. Later, it can be learned that this number is called "increasing root" and it is a root that does not exist.

If we multiply, when the equation has two or more solutions, one of which is x=2, then we are equivalent to multiplying both sides of the equation by 0 at the same time. Then when this root is established, the situation will be multiplied by us, which is a phenomenon of "root dropping", that is to say, when contemporary number expressions are multiplied at the same time, the solved equation will lose a root.

So ②: "When both sides of an equation are multiplied by the same number (or divided by the same number that is not zero), the result is still an equation." This statement is correct.

Of course, when solving practical problems, both sides can be multiplied or divided by an algebraic expression at the same time.

When both sides are multiplied by an algebraic expression at the same time, it is finally necessary to check whether the value that makes this algebraic expression zero is the follow of the original equation.

When both sides are divisible by an algebraic expression at the same time, it is finally necessary to check whether there is a root in the solved value that makes the algebraic expression zero, and if there is one, discard it directly.