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)？ .