Fan Chaoke of Tang Xiao Mathematics Group
The addition and subtraction within 100 is the most difficult. Generally, when we calculate this kind of problems, we will use the vertical calculation method, which is relatively simple but not the simplest method. Today, I will teach you a simple calculation method of 100 carry addition and abdication subtraction.
In fact, whether it is carry addition or abdication subtraction, the most basic foundation is the ten-point method, so we must master the ten-point method first. The formula is: one equals nine, two equals eight, three equals seven, four equals six, and five or five is just enough.
Simple calculation method of carry addition
No matter how much you add, the most basic principle is the addition principle within 20. The fast formula of carry addition within 20 is: add 90% minus one, add 80% minus two, add 70% minus three, and add 60% minus four. Because addition has commutative law, we only need to remember these words. In addition, within 100, first observe the two digits, find out the larger one, and get the answer quickly according to the formula calculation.
Example 1: 26+39 =
We observed that the two digits are 6 and 9, respectively, and 6+9 is greater than 10, which requires rounding, and the larger digit is 9, so the ten digits of the answer obtained by applying "several plus nine plus ten minus one" are 2+3+ 1=6, and the single digit is 6 minus 1 equal to 5, so the answer is 65.
Example 2: 38+54=
We observed that the two digits are 8 and 4, respectively, and 8+4 is greater than 10, which requires rounding, and the larger digit is 8, so the ten digits of the answer obtained by applying "several plus eight plus ten minus two" are 3+5+ 1=9, and the single digit is 4 minus 2 equals 2, so the answer is 92.
Let's do some exercises to see who can calculate quickly and accurately:
9+5= 3+8= 26+55= 34+49= 67+25= 58+ 19=
39+25= 26+38= 19+4 1= 28+47= 43+39= 36+56=
Second, the calculation method of short answer questions of abdication subtraction
The abdication subtraction within 100 is also based on the abdication subtraction within 20, and the speed of abdication subtraction.
The formula is as follows: ten plus one minus nine, ten plus two minus eight, ten plus three minus seven, ten plus four minus six, ten plus five minus five, ten plus six minus four, ten plus seven minus three, ten plus eight minus two, and ten plus nine minus one. Because the positions of subtree and subtree in subtraction cannot be interchanged, in subtraction, two digits are observed first. When the subtree is larger than the single digits of the subtree, the answer can be calculated quickly by selecting the formula according to each digit of the subtree.
Example 3: 54-29 =
Through observation, we found that the unit of the minuend is 4 and the unit of the minuend is 9,4.
Example 4: 63- 16 =
Through observation, we found that the unit of the minuend is 3 and the unit of the minuend is 6,3.
Let's do some exercises to see who can calculate quickly and accurately:
14—9= 15—6= 23— 18= 43— 19= 54—35=
3 1—22= 45—28= 88—39= 72—36= 66—38=
Students, today we mainly learned the fast algorithms of carry addition and abdication subtraction within 100. Quick calculation, also called quick calculation, is the perfect combination of oral calculation and written calculation. Mainly depends on students to master the law of fast calculation, strong sense of number, logarithmic thinking and memory, and the calculation method of number is calculated through oral calculation combined with simple pen calculation. There are many quick calculation methods in our primary school mathematics. I hope that in the future study and problem solving, we will give full play to our intelligence and sum up more quick calculation methods to make our math learning more simple and interesting.