Modulo 9 prooj Study the principle of the modulo 9 prooj for the addition, multiplication and division operations of decimal numbers (base 10).

1. Show that searching for the dass of a number is equivalent to searching for the dass of the sum of the dasses of each figure of the number. This process is iterative.

2. Use the modulo 9 proof to check if the following operations are correct:

• 189 + 47 = 236

• 189x47=8867

• 189 - 47 = 144

• 189 - 47 = 97 3. Show with an example that the modulo 9 proof is false for division?

4. What is the class of non-detectable errors with this code?