Prove that $3^{2n} +7$ is divisible by 8
$begingroup$
Prove by induction that $3^{2n} +7$ is divisible by 8 for $n in Bbb N$
So I think I have completed this proof but it doesn't seem very thorough to me - is my proof valid?
If $n=1$ then $3^{2n} +7 = 16 = 2(8)$ so true when $n=1$
Assume true for $n=k$ so $$ 8vert 3^{2k} +7$$
If $n=k+1$
$$3^{2(k+1)} +7$$
$$3^{2k+2} +7$$
If $3^{2n} +7$ is divisble by 8 then $3^{2n} +7=8A$ where $A in Bbb Z$, and thus $3^{2k+2} +7=8B$ where $B in Bbb Z$
$$3^2 times 3^{2k}+7$$
$$3^2 times (8A)=72A$$
$$72A =8(9A)=9B$$
So by induction $3^{2n} +7$ is divisibe by 8 $forall n in Bbb N$
elementary-number-theory induction divisibility
$endgroup$
add a comment |
$begingroup$
Prove by induction that $3^{2n} +7$ is divisible by 8 for $n in Bbb N$
So I think I have completed this proof but it doesn't seem very thorough to me - is my proof valid?
If $n=1$ then $3^{2n} +7 = 16 = 2(8)$ so true when $n=1$
Assume true for $n=k$ so $$ 8vert 3^{2k} +7$$
If $n=k+1$
$$3^{2(k+1)} +7$$
$$3^{2k+2} +7$$
If $3^{2n} +7$ is divisble by 8 then $3^{2n} +7=8A$ where $A in Bbb Z$, and thus $3^{2k+2} +7=8B$ where $B in Bbb Z$
$$3^2 times 3^{2k}+7$$
$$3^2 times (8A)=72A$$
$$72A =8(9A)=9B$$
So by induction $3^{2n} +7$ is divisibe by 8 $forall n in Bbb N$
elementary-number-theory induction divisibility
$endgroup$
$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03
add a comment |
$begingroup$
Prove by induction that $3^{2n} +7$ is divisible by 8 for $n in Bbb N$
So I think I have completed this proof but it doesn't seem very thorough to me - is my proof valid?
If $n=1$ then $3^{2n} +7 = 16 = 2(8)$ so true when $n=1$
Assume true for $n=k$ so $$ 8vert 3^{2k} +7$$
If $n=k+1$
$$3^{2(k+1)} +7$$
$$3^{2k+2} +7$$
If $3^{2n} +7$ is divisble by 8 then $3^{2n} +7=8A$ where $A in Bbb Z$, and thus $3^{2k+2} +7=8B$ where $B in Bbb Z$
$$3^2 times 3^{2k}+7$$
$$3^2 times (8A)=72A$$
$$72A =8(9A)=9B$$
So by induction $3^{2n} +7$ is divisibe by 8 $forall n in Bbb N$
elementary-number-theory induction divisibility
$endgroup$
Prove by induction that $3^{2n} +7$ is divisible by 8 for $n in Bbb N$
So I think I have completed this proof but it doesn't seem very thorough to me - is my proof valid?
If $n=1$ then $3^{2n} +7 = 16 = 2(8)$ so true when $n=1$
Assume true for $n=k$ so $$ 8vert 3^{2k} +7$$
If $n=k+1$
$$3^{2(k+1)} +7$$
$$3^{2k+2} +7$$
If $3^{2n} +7$ is divisble by 8 then $3^{2n} +7=8A$ where $A in Bbb Z$, and thus $3^{2k+2} +7=8B$ where $B in Bbb Z$
$$3^2 times 3^{2k}+7$$
$$3^2 times (8A)=72A$$
$$72A =8(9A)=9B$$
So by induction $3^{2n} +7$ is divisibe by 8 $forall n in Bbb N$
elementary-number-theory induction divisibility
elementary-number-theory induction divisibility
edited Jan 9 at 21:35
Maria Mazur
49.7k1361124
49.7k1361124
asked Jan 9 at 18:54
H.LinkhornH.Linkhorn
485213
485213
$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03
add a comment |
$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03
$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
No, it is not correct. It sholud be done something like this:
$$3^{2k}= 8A-7$$ sobegin{eqnarray} 3^{2k+2}+7 &=& 9cdot 3^{2k}+7\ &=& 9cdot (8A-7)+7 \ &=& 72A-56\ &=& 8(9A-7)end{eqnarray}
$endgroup$
add a comment |
$begingroup$
An option:
Step $n+1$:
$3^{2n+2}+7 =9 cdot 3^{2n} +7=$
$(8+1)cdot 3^{2n} +7=$
$(3^{2n}+7) +8;$
The first term is divisible by $8$ by hypothesis, so is the second term.
$endgroup$
add a comment |
$begingroup$
Posting this community wiki answer as a protest - instructors should not ask for proofs by induction when there are much more straightforward ones like
$$
3^{2n} + 7 = 9^n + 7 equiv 1^n + 7 equiv 0 pmod{8} .
$$
Save induction for questions where it's really important.
$endgroup$
add a comment |
$begingroup$
You've got the right idea. Here's how to view the inductive step intuitively as multiplication
$$begin{align}
3^{large 2k} &= color{#c00}{-7} + 8aqquad {rm i.e.} P(k)\
times 3^{large 2} &= color{#c00} 1 + 8phantom{I_{I_{I_I}}}\
hline
Rightarrow 3^{large 2(k+1)} &= {-7} + 8(cdots) {rm i.e.} P(k!+!1)
end{align}qquad $$
Renark $ $ Note $, -7! +! 8a = 1! +! 8(a!-!1) $ so viewed this way, instead of $, color{#c00}{-7 times 1},$ we get $,1times 1,,$ so the above boils down to $, (1+8j)(1+8k) = 1+8n,,$ i.e. $bmod 8!: 1^2equiv 1, $ so the result boils down to $,9equiv 1,Rightarrow, 9^nequiv 1^nequiv 1,$ by the Congruence Power Rule.
$endgroup$
add a comment |
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
No, it is not correct. It sholud be done something like this:
$$3^{2k}= 8A-7$$ sobegin{eqnarray} 3^{2k+2}+7 &=& 9cdot 3^{2k}+7\ &=& 9cdot (8A-7)+7 \ &=& 72A-56\ &=& 8(9A-7)end{eqnarray}
$endgroup$
add a comment |
$begingroup$
No, it is not correct. It sholud be done something like this:
$$3^{2k}= 8A-7$$ sobegin{eqnarray} 3^{2k+2}+7 &=& 9cdot 3^{2k}+7\ &=& 9cdot (8A-7)+7 \ &=& 72A-56\ &=& 8(9A-7)end{eqnarray}
$endgroup$
add a comment |
$begingroup$
No, it is not correct. It sholud be done something like this:
$$3^{2k}= 8A-7$$ sobegin{eqnarray} 3^{2k+2}+7 &=& 9cdot 3^{2k}+7\ &=& 9cdot (8A-7)+7 \ &=& 72A-56\ &=& 8(9A-7)end{eqnarray}
$endgroup$
No, it is not correct. It sholud be done something like this:
$$3^{2k}= 8A-7$$ sobegin{eqnarray} 3^{2k+2}+7 &=& 9cdot 3^{2k}+7\ &=& 9cdot (8A-7)+7 \ &=& 72A-56\ &=& 8(9A-7)end{eqnarray}
answered Jan 9 at 18:57
Maria MazurMaria Mazur
49.7k1361124
49.7k1361124
add a comment |
add a comment |
$begingroup$
An option:
Step $n+1$:
$3^{2n+2}+7 =9 cdot 3^{2n} +7=$
$(8+1)cdot 3^{2n} +7=$
$(3^{2n}+7) +8;$
The first term is divisible by $8$ by hypothesis, so is the second term.
$endgroup$
add a comment |
$begingroup$
An option:
Step $n+1$:
$3^{2n+2}+7 =9 cdot 3^{2n} +7=$
$(8+1)cdot 3^{2n} +7=$
$(3^{2n}+7) +8;$
The first term is divisible by $8$ by hypothesis, so is the second term.
$endgroup$
add a comment |
$begingroup$
An option:
Step $n+1$:
$3^{2n+2}+7 =9 cdot 3^{2n} +7=$
$(8+1)cdot 3^{2n} +7=$
$(3^{2n}+7) +8;$
The first term is divisible by $8$ by hypothesis, so is the second term.
$endgroup$
An option:
Step $n+1$:
$3^{2n+2}+7 =9 cdot 3^{2n} +7=$
$(8+1)cdot 3^{2n} +7=$
$(3^{2n}+7) +8;$
The first term is divisible by $8$ by hypothesis, so is the second term.
answered Jan 9 at 19:08
Peter SzilasPeter Szilas
11.7k2822
11.7k2822
add a comment |
add a comment |
$begingroup$
Posting this community wiki answer as a protest - instructors should not ask for proofs by induction when there are much more straightforward ones like
$$
3^{2n} + 7 = 9^n + 7 equiv 1^n + 7 equiv 0 pmod{8} .
$$
Save induction for questions where it's really important.
$endgroup$
add a comment |
$begingroup$
Posting this community wiki answer as a protest - instructors should not ask for proofs by induction when there are much more straightforward ones like
$$
3^{2n} + 7 = 9^n + 7 equiv 1^n + 7 equiv 0 pmod{8} .
$$
Save induction for questions where it's really important.
$endgroup$
add a comment |
$begingroup$
Posting this community wiki answer as a protest - instructors should not ask for proofs by induction when there are much more straightforward ones like
$$
3^{2n} + 7 = 9^n + 7 equiv 1^n + 7 equiv 0 pmod{8} .
$$
Save induction for questions where it's really important.
$endgroup$
Posting this community wiki answer as a protest - instructors should not ask for proofs by induction when there are much more straightforward ones like
$$
3^{2n} + 7 = 9^n + 7 equiv 1^n + 7 equiv 0 pmod{8} .
$$
Save induction for questions where it's really important.
answered Jan 9 at 19:12
community wiki
Ethan Bolker
add a comment |
add a comment |
$begingroup$
You've got the right idea. Here's how to view the inductive step intuitively as multiplication
$$begin{align}
3^{large 2k} &= color{#c00}{-7} + 8aqquad {rm i.e.} P(k)\
times 3^{large 2} &= color{#c00} 1 + 8phantom{I_{I_{I_I}}}\
hline
Rightarrow 3^{large 2(k+1)} &= {-7} + 8(cdots) {rm i.e.} P(k!+!1)
end{align}qquad $$
Renark $ $ Note $, -7! +! 8a = 1! +! 8(a!-!1) $ so viewed this way, instead of $, color{#c00}{-7 times 1},$ we get $,1times 1,,$ so the above boils down to $, (1+8j)(1+8k) = 1+8n,,$ i.e. $bmod 8!: 1^2equiv 1, $ so the result boils down to $,9equiv 1,Rightarrow, 9^nequiv 1^nequiv 1,$ by the Congruence Power Rule.
$endgroup$
add a comment |
$begingroup$
You've got the right idea. Here's how to view the inductive step intuitively as multiplication
$$begin{align}
3^{large 2k} &= color{#c00}{-7} + 8aqquad {rm i.e.} P(k)\
times 3^{large 2} &= color{#c00} 1 + 8phantom{I_{I_{I_I}}}\
hline
Rightarrow 3^{large 2(k+1)} &= {-7} + 8(cdots) {rm i.e.} P(k!+!1)
end{align}qquad $$
Renark $ $ Note $, -7! +! 8a = 1! +! 8(a!-!1) $ so viewed this way, instead of $, color{#c00}{-7 times 1},$ we get $,1times 1,,$ so the above boils down to $, (1+8j)(1+8k) = 1+8n,,$ i.e. $bmod 8!: 1^2equiv 1, $ so the result boils down to $,9equiv 1,Rightarrow, 9^nequiv 1^nequiv 1,$ by the Congruence Power Rule.
$endgroup$
add a comment |
$begingroup$
You've got the right idea. Here's how to view the inductive step intuitively as multiplication
$$begin{align}
3^{large 2k} &= color{#c00}{-7} + 8aqquad {rm i.e.} P(k)\
times 3^{large 2} &= color{#c00} 1 + 8phantom{I_{I_{I_I}}}\
hline
Rightarrow 3^{large 2(k+1)} &= {-7} + 8(cdots) {rm i.e.} P(k!+!1)
end{align}qquad $$
Renark $ $ Note $, -7! +! 8a = 1! +! 8(a!-!1) $ so viewed this way, instead of $, color{#c00}{-7 times 1},$ we get $,1times 1,,$ so the above boils down to $, (1+8j)(1+8k) = 1+8n,,$ i.e. $bmod 8!: 1^2equiv 1, $ so the result boils down to $,9equiv 1,Rightarrow, 9^nequiv 1^nequiv 1,$ by the Congruence Power Rule.
$endgroup$
You've got the right idea. Here's how to view the inductive step intuitively as multiplication
$$begin{align}
3^{large 2k} &= color{#c00}{-7} + 8aqquad {rm i.e.} P(k)\
times 3^{large 2} &= color{#c00} 1 + 8phantom{I_{I_{I_I}}}\
hline
Rightarrow 3^{large 2(k+1)} &= {-7} + 8(cdots) {rm i.e.} P(k!+!1)
end{align}qquad $$
Renark $ $ Note $, -7! +! 8a = 1! +! 8(a!-!1) $ so viewed this way, instead of $, color{#c00}{-7 times 1},$ we get $,1times 1,,$ so the above boils down to $, (1+8j)(1+8k) = 1+8n,,$ i.e. $bmod 8!: 1^2equiv 1, $ so the result boils down to $,9equiv 1,Rightarrow, 9^nequiv 1^nequiv 1,$ by the Congruence Power Rule.
edited Jan 9 at 19:21
answered Jan 9 at 19:05
Bill DubuqueBill Dubuque
213k29196654
213k29196654
add a comment |
add a comment |
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$begingroup$
Not following. Yes, you may assume (inductively) that $3^{2n}+7=8A$ and yes, you want to argue that this implies that $3^{3k+2}+7=8B$, but where do you argue that? It is simply not true that $3^2times 3^{2k}+7=3^2times (8A)$.
$endgroup$
– lulu
Jan 9 at 18:57
$begingroup$
Do you know modular arithmetic / congruences?
$endgroup$
– Bill Dubuque
Jan 9 at 18:59
$begingroup$
You made an error. Note that $3^2cdot 3^{2k}+7 color{red}{neq 3^2cdot left(3^{2k}+7right)}$.
$endgroup$
– KM101
Jan 9 at 18:59
$begingroup$
Not relevant to the proof by induction, but it is worth noting that there is a very simple proof of this fact using basic binomial expansion: $3^{2n}+7=9^n+7=(1+8)^n+7$ and after expansion all your terms are divisible by $8$.
$endgroup$
– AlephNull
Jan 9 at 19:03