Addition and subtraction formula within 10: think of 1 when you see 9, 2 when you see 8, 3 when you see 7, 4 when you see 6, and change the two numbers first when you see the decimal system; The calculation method of 3-digit addition and subtraction: the digits are aligned, adding and subtracting from single digits, adding one for every ten digits, and subtracting if it is not enough, and retreating from the previous digit to ten.

Addition formula within 10:

1+ 1=2

2+ 1=3 1+2=3

3+ 1=42+2=4 1+3=4

4+ 1=53+2=52+3=5 1+4=5

5+ 1=64+2=63+3=62+4=6 1+5=6

6+ 1=75+2=74+3=73+4=72+5=7 1+6=7

7+ 1=86+2=85+3=84+4=83+5=82+6=8 1+7=8

8+ 1=97+2=96+3=95+4=94+5=93+6=92+7=9 1+8=9

9+ 1= 108+2= 107+3= 106+4= 105+5= 104+6= 103+7= 102+8= 10 1+9= 10

The subtraction formula within 10:

2- 1= 1

3- 1=23-2= 1

4- 1=34-2=24-3= 1

5- 1=45-2=35-3=25-4= 1

6- 1=56-2=46-3=36-4=26-5= 1

7- 1=67-2=57-3=47-4=37-5=27-6= 1

8- 1=78-2=68-3=58-4=48-5=38-6=28-7= 1

9- 1=89-2=79-3=69-4=59-5=49-6=39-7=29-8= 1

10- 1=9 10-2=8 10-3=7 10-4=6 10-5=5 10-6=4 10-7=3 10-8=2 10-9= 1