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How to work out the fourth grade mathematics quickly?
1, familiar with the fourth grade mathematical formula, understand the calculation law, you can quickly calculate the number. Formula: two addends exchange positions, and the sum is unchanged. This is additive commutative law. a+b=b+a? .

2. Add the first two numbers, or add the last two numbers first, and the sum is unchanged. This is the associative law of addition. (a+b)+c=a+(b+c)? .

3. Swap the positions of two factors, and the product remains unchanged. This is the so-called multiplication commutative law. a×b=b×a? .

4. Multiply the first two numbers, or multiply the last two numbers first, and the product remains unchanged. This is the so-called law of multiplication and association. (a×b)×c=a×(b×c)? .