计算20以内的退位减法时,我们可以用“破十法”与“平十法”的方法来想。例如,15-7=□。我们可以通过用小棒演示来理解“破十法”与“平十法”的道理。一、“破十法”。这种算法是先分开一捆小棒,再从10根小棒中拿走7根,然后把余下的小棒合起来。也就是先算10-7=3,再算3+5=8,所以15-7=8。口算过程可以用以下算式表示:15- When calculating the abatement subtraction within 20, we can use the method of “breaking 10” and “flat 10” to think. For example, 15-7 = □. We can understand the truth of “broken ten ” and “flat ten ” by using a small stick demonstration. First, “broken ten law ”. This algorithm is to first separate a bundle of small sticks, and then take away from the 10 small sticks 7, and then the remaining small stick together. That is the first count 10-7 = 3, then count 3 +5 = 8, so 15-7 = 8. The calculation process can be expressed by the following formula: 15-