The decimal method is to divide the subtree into two numbers. After subtracting the first number from the subtree, it will be equal to 10, and then subtract the second number from 10 to get the final result.

For example, in the calculation of 9+7, we should first divide 7 into 1 and 6, because 9 just lacks 1, so we should divide 1 from 7. So 9+7 should be calculated in two steps, 9+ 1= 10 and 10+6= 16, and finally 9+7 = 16 is calculated.

Therefore, we must remember the formulas "9 to 1", "8 to 2", "7 to 3", "6 to 4" and "5 to 5" clearly.

Extended data

The addition and subtraction operation within 20 is one of the most important contents in the first grade mathematics. Four operation methods are used in the operation, including ten addition, ten break, ten leveling and ten borrowing, of which two methods are the most commonly used.

When doing operations within 20 minutes, you should also pay attention to the following two issues.

1, abdication subtraction: abdication subtraction should be kept in mind and started from the unit; Who is not enough to retire, don't forget to add ten to the standard; If the interval is 1, it is best to remember that 0 becomes 10.

2, continuous abdication subtraction: see 0, go forward and see which one has it; Borrowed and walked back, 0 is a bit like 9.