Prove 24|2*7^n+3*5^n-5
I showed true for n=1:
2*7+3*5-5=14+15-5=24
Thus divisible by 24
Assume true for all k>=1. Need to prove true for n=k+1
Then 2*7^(k+1)+3*5^(k+1)-5=14*7^k+15*5^k-5
I get this far and now I get stuck simplifying enough to show it's true.