The parentheses are preceded by the invariant symbol of the plus sign!

This is the symbol of negative sign. Increase and decrease, decrease and increase.

There is nothing before the brackets. It's actually a plus sign! Invariant symbol!

2.

Multiplication: Multiply the number outside brackets by the number inside brackets, while keeping the plus sign and minus sign in front of the number inside brackets.

Except: the minus sign is in front of the brackets, and the sign should be changed when the brackets are removed.

3.

It is preceded by a multiplication symbol and enclosed in parentheses by an invariant symbol. Parentheses should be preceded by a division symbol and followed by a minus sign to form a division symbol. Increase or decrease.

4. Operating rules:

① commutative law of addition: a+b = b+a;

② the associative law of addition: (a+b)+c = a+(b+c);

③ Multiplicative commutative law: AB = BA

④ Multiplicative associative law: (ab) c = a (BC);

⑤ Distribution law of multiplication to addition: A (B+C) = AB+AC;

Note: Division has no distribution law.