17 can be divided into 10 and 7, then use 10-9= 1, and then add 7+ 1=8 to the original 7.

So 17-9=8.

Or split 9 into 7 and 2, and let 17 subtract 7 and then 2.

2. Use the counting method

Write down the number from 1 to 17, then subtraction means counting forward to the ninth, and addition means counting backward.

This method is suitable for addition, subtraction and testing of small numbers.