The formula of this number should be: (9+3)÷2=6 books.
This is an effective application of addition, subtraction, multiplication and division and elementary arithmetic. We must be familiar with the law in order to obtain fast and accurate calculation results.
Mathematical fast calculation method:
Fast addition calculation
1. Rounding and addition
Rounding-up addition is the rounding-up addition difference method, and the rounding-up difference can be calculated quickly. When 8+7= 15 is calculated, first add 8 to 10 8 plus 2 equals 10 7 minus 2 equals 5 10+5 = 15.
If 17+9 = 26, the calculation program is 17+3=20 9-3=6 20+6=26.
2. Complement addition
Complement addition is fast, mainly because there is no trouble of carrying it bit by bit. The complement is the sum of two numbers 10 100 1000 and so on. 8+2= 10 78+22= 100 8 is the complement of 2, 2 is also the complement of 8, 78 is the complement of 22, and 22 is also the complement of 78.
The method of adding with complement is to add 1 to ten digits and subtract one digit. For example, when calculating 6+8= 14, add 1 to the decimal number of 6 to become 16, and then subtract 8' s complement 2 from 16 to get 14.
Such as 6+7= 13, 6+ 10= 16 and then 16-3= 13.
Such as 27+8=35 27+ 10=37 37-2=35.
Such as 25+85 =11025+100 =125-15 =10.
Such as 867+898 =1765 867+1000 =18671867-102 =1765.
3. Add transposition positions
There is a quick calculation method for the exchange of two decimal places: ten digits plus one digit, and one digit plus two digits, and two digits plus the center.
For example, 6 1+ 16=77 and the calculation program is 6+ 1 = 77. 7 is a single digit, and the sum is a double number, that is, two 7,61+16 = 77, and another example is 83+38 = 1265438.
The calculation program is 8+3 =111,which is a two-digit number. When the two numbers are added, 1+ 1 = 2 is in the middle of 1 1, and 12658 is obtained.
Fast calculation of subtraction
One. Complement subtraction of two digits MINUS one
Two digits minus one digit's complement subtraction is: ten digits minus 1, one digit plus one's complement. For example, 15-8 = 7, 15 minus 10 equals 5, and the complement of 5 plus 8 equals 7.
2. Multi-bit complement subtraction
Complement subtraction is 1 minus the complement, and three digits minus two digits: one hundred digits minus 1, and ten digits plus the complement, such as 268-89 = 179. The calculation program is 268 minus 100 equals 168 and 168 plus 89' s complement.