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Sign-changing law of multiplication distribution rate in primary school mathematics
1. Parentheses are preceded by a "+". Remove the brackets and the "+"in front of them, and the symbols of the items in brackets remain unchanged.

There is a "-"before the brackets. Remove the brackets and the preceding "-"sign, and the symbol of each item in brackets should be changed to the opposite symbol.

In fact, the basis of the rule of removing brackets is multiplication and division.

Note: Pay attention to the symbol before brackets, which is the basis for whether the items in brackets change their signs after removing brackets.

When deleting brackets, the symbol before brackets should be deleted together with brackets.

It should be noted that when there is a "-"sign in front of the brackets, the symbols of the items in brackets will change after the brackets are removed. You can't just change the symbol of the first item or the first few items in brackets and forget to change other symbols.

If there is a numerical factor in front of the brackets, before removing the brackets, multiply the number with the items in brackets by multiplication and division to avoid mistakes.

When you encounter multiple layers of brackets, you usually remove the brackets from inside to outside, and you can also count the number of "-"from outside to inside.

example

Example 1: Remove brackets first, and then merge similar items.

( 1)5a-(2a-4b); (2) 2x2+3(2x-x2) (3) 4+7(2x+8x)

Solution: (1)5a-(2a-4b)

=5a-2a+4b

=3a+4b

(2)2x2+3(2x-x2)

=2x2+6x-3x2

=-x2+6x

(3)4+7(2x+8x)

=4+ 14x+56x

= 4+70 times