Question 14
Consider
$$A=37^n+71^n-2^n-1,$$
$$B=n(n^2-6)^2(n^2-1)-n^3(n-1)(n+1),$$
$$C=1+2+\cdots+(21n+1).$$
Which expressions are divisible by 21 for every natural number $n$?
$$A=37^n+71^n-2^n-1,$$
$$B=n(n^2-6)^2(n^2-1)-n^3(n-1)(n+1),$$
$$C=1+2+\cdots+(21n+1).$$
Which expressions are divisible by 21 for every natural number $n$?