For example, 2*9= 18 can be regarded as the addition of two 9s (9+9) or the addition of nine 2s (2+2+2+2+2+2+2+2+2+2).
2. Multiply the integer 10 or 100 by one digit. First, remove the zero after the integer 10 or 100, and then multiply it by another number to get a product. Then, if there are several zeros after the factor, add several zeros after the product.
Such as 20*7= 140. First, take 20 as the product of 2 and 7 to get 14, and add a 0 after 14 to get 140.
Extended data:
Law of multiplication in primary schools
1, the commutative law of multiplication: two numbers are multiplied, and the positions of two factors are exchanged, and the product is unchanged, which is called the commutative law of multiplication.
Expression: a*b = b*a
2. Law of multiplication: When three numbers are multiplied, the first two numbers are multiplied by the third number, or the last two numbers are multiplied by the first number, and the product remains unchanged. This is the so-called law of multiplication and association.
Expression: a*b*c = a*(b*c)
3. Multiplication and distribution law: Multiplying the sum (or difference) of two numbers with a number is equivalent to multiplying these two numbers with this number respectively, and then adding (or subtracting) the two products. This is the so-called law of multiplication and division. ?
Expression: (a+b) * c = a * c+b * c (a-b) * c = a * c-b * c.