My Math Forum Induction again

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 April 8th, 2014, 03:11 AM #1 Senior Member     Joined: Oct 2013 From: Far far away Posts: 422 Thanks: 18 Induction again Prove that (4^n) - 1 is divisible by 3 My attempt: 1) let n = 1 (4^1) - 1 = 4 - 1 = 3 3 is divisible by 3. So true for n = 1 2) Assume it is true for n = k So (4^k) - 1 is divisible by 3 3) let n = k + 1 $4^{k+1}-1=(4^k)(4)-1=(4^k)(3+1)-1=3(4^k)+[4^k+1]$ but $3(4^k)$ and $4^k+1$ are divisible by 3 so $4^{k+1}$ is divisible by 3 Therefore for all n 4^n - 1 is divisible by 3 Is this proof acceptable? The third step doesn't look like induction..thanks
April 8th, 2014, 03:19 AM   #2
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Quote:
 Originally Posted by shunya 3) let n = k + 1 $4^{k+1}-1=(4^k)(4)-1=(4^k)(3+1)-1=3(4^k)+[4^k+1]$ but $3(4^k)$ and $4^k+1$ are divisible by 3
$4^k+1$ should be replaces by $4^k-1$ (two times). Otherwise, the proof is correct. For clarity, it would be nice to say that $3\mid(4^k-1)$ by induction hypothesis.

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