Definition: if there are two two digits, and the number of ten and ten of any one digit is the number of ten and ten of another digit respectively, then such two digits are called "mutually friendly two digits" (1) Try to explain the reason why the difference between two digits must be a multiple of 9; (2) Based on the above experience, what conclusions can you draw?

Definition: if there are two two digits, and the number of ten and ten of any one digit is the number of ten and ten of another digit respectively, then such two digits are called "mutually friendly two digits" (1) Try to explain the reason why the difference between two digits must be a multiple of 9; (2) Based on the above experience, what conclusions can you draw?

Let one of the numbers be 10x + y (denote x for ten digits and y for each digit),
1)
(10x+y)-(x+10y)=9(x-y)
So it's a multiple of nine
2)
(10x+y)+(x+10y)=11(x+y)
The sum of two numbers is a multiple of 11