F.4 Mathematical induction

Prove by mathematical induction that: (for any positive integers n)
17) [(6n - 1) * 7^n] + 1 is divisible by 36.
18) n^4 + 2n^3 - n^2 + 14n is divisible by 8.
19) (7n + 1)*6^n + (-1)^(n+1) is divisible by 49.
20) (5n + 24)*6^n + 1 is divisible by 25.