Common mistakes:
- On 3.56, many students proved the base case for n=3. But since the question is to prove for all natural numbers, you should prove the base case for n=1, 2.
- On 12.19, many students forgot the base case for n=0.
- On 4.13, Some students get the digits for x as a-c-1, 9, 10-(a-c) without the assumption that a>c.