For example: 86*84=7224
(8+ 1)*8=72, 6*4=24, which is 7224.
4 1*49=2009
(4+ 1)*4=20, 1*9=9. If it is less than 10, it is 0, which is 09, so the final result is 2009.
2. Ten digits are complementary, and two digits with the same digits are multiplied. Formula: multiply ten bits by one bit and add one bit, and multiply one bit and write it at the back (less than 10 to make up 0).
For example: 64x44=28 16
6×4+4=28, 4×4= 16, which is 28 16.
73×33=2409
7×3+3=24, 3×3=9, less than 10 is 0, which is 09, so the result is 2409.
Similarly, the square of 5 1-59 can also be calculated in this way. For example, the square of 56 is equal to 3 136, 5×5+6=3 1, 6×6 = 36, that is, 3 136.
3. Ten digits and one digit of a number are complementary, and two numbers with the same number are multiplied. Formula: Add one to the ten digits of the complement, multiply it by the high digit of another number, and then write two digits to multiply it, which is the final product (less than 10 plus 0).
Example: 46x77=3542
(4+ 1)x7=35, 6x7=42, which is 3542.
9 1x33=3003
(9+ 1)×3=30, 1×3=3, if less than 10, it is 0, that is, the result is 3003.
73×66666666=48666666 18
(7+ 1)x6=48, the middle six sixes are written without multiplication, and 3x6= 18 is written later, which is 48666618. As long as the ten digits of one number complement each other, no matter how much the other number is the same, just calculate the highest digit and single digit, and copy in the middle.
4. Multiply any number with 1 1. Formula: from left to right, write as much as you can in the high position, then add two and two in turn, carry more than ten, and finally write one bit.
For example: 32618372x11= 358802092.
Write 3 in the high order 3, and then write 3+2=5, 2+6=8, 6+ 1 = 7, 1+8 = 9, 8+3 = 1/(write 1/as/kloc.
5. The operation of multiplying by ten. Formula: the sum of one number plus the tail of another number multiplied by ten, plus the mantissa multiplication is the final result.
For example: 14x 13= 182
( 14+3)× 10= 170,4×3= 12, 170+ 12= 182
18x 17=306
( 18+7)x 10=250，8×7=56，250+56=306
Similarly, you can also use this method to find the square of 1 1 to 19.
6. Single-digit multiplication of1 Formula: The product multiplied by the first digit is connected with the sum of the first digit (less than 10 plus 0), and then connected with the product of mantissa.
For example: 41x 31=1271
4×3= 12, 4+3 = 7, 1x 1 = 1, that is, 127 1.
5 1×8 1=4 13 1
5×8=40, 5+8= 13 (write 3 as 1, with 4 1 in front), followed by 1x 1= 1, which is 4/kloc-0.
7. One hundred times one hundred. Formula: Add the mantissa of one number to another number, and then connect the products of mantissas (less than 10 to make up 0).
For example:103x105 =10815.
103+5= 108, 3x5= 15, namely 108 15.
102 x 103 = 10506
102+3= 105, 2x3=6. If it is less than 10, it is 0, that is, 10506.
Similarly, you can also use this method to find the square of 10 1 to 109. For example, the square of 108 is 1 1664,108+8 =16 and then 8×8=64, and the result is1/kloc-0.