What is the smallest number n> 5 so that 5 ^ n ends with “3125”?
$begingroup$
What is the smallest number n> 5 so that 5 ^ n ends with "3125"?
What other examples are there?
calculus
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
$begingroup$
What is the smallest number n> 5 so that 5 ^ n ends with "3125"?
What other examples are there?
calculus
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
1
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
1
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
4
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55
add a comment |
$begingroup$
What is the smallest number n> 5 so that 5 ^ n ends with "3125"?
What other examples are there?
calculus
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
What is the smallest number n> 5 so that 5 ^ n ends with "3125"?
What other examples are there?
calculus
calculus
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked Mar 20 at 19:59
Catherine Cooper Catherine Cooper
243
243
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Catherine Cooper is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
1
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
4
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55
add a comment |
1
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
1
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
4
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55
1
1
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
1
1
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
4
4
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
So, we are looking for all $n>5$ for which $5^nequiv 3125=5^5mod 10000$.
Note that the following equivalence holds for $n>5$:$$5^nequiv 5^5mod 10000\iff \5^{n-4}equiv 5mod 16\iff\5^{n-5}equiv 1mod 16$$Define $mtriangleq n-5ge 1$. Then all the $m$s for which $5^{m}equiv 1mod 16$ holds are $$m=4kquad,quad kin Bbb N$$this is because $5^4=625equiv 1mod 16$ and therefore $$5^{4k}equiv5^{4k-4}equivcdots equiv 5^{4}equiv 1mod 16$$
Conclusion
All $n>5$s for which $5^n$ ends up with $3125$ can be found from $$n=4k+5quad,quad kin Bbb N$$ and the smallest such $n$ is 9.
$endgroup$
add a comment |
$begingroup$
Hint: $5^n equiv 5^5 mod 10^4$ if and only if $5^n equiv 5^5 mod 2^4$. What is the multiplicative order of $5$ mod $16$?
$endgroup$
add a comment |
$begingroup$
Well
$$5^9=1953125$$
so the answer is $9$. In fact
$$5^nequiv 5^{n-4} mod{10^4}$$
For $nge 8$, so any value of $5^{5+4k}$ where $kinmathbb{N}$ has the last four digits $3125$.
$endgroup$
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
add a comment |
$begingroup$
Hint $, 5^{large 5+N}! bmod 10^{large 4} = 5^{large 5}(5^{largecolor{#c00} N}! bmod 2^{large 4}).,$ Now recall $, begin{align} 5, &equiv 1!pmod{! color{#c00}4} \ Rightarrow 5^{largecolor{#c00} 4}!&equiv 1^{largecolor{#c00} 4}!!!! pmod{!color{#c00}4^{large 2}}end{align}$
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
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votes
$begingroup$
So, we are looking for all $n>5$ for which $5^nequiv 3125=5^5mod 10000$.
Note that the following equivalence holds for $n>5$:$$5^nequiv 5^5mod 10000\iff \5^{n-4}equiv 5mod 16\iff\5^{n-5}equiv 1mod 16$$Define $mtriangleq n-5ge 1$. Then all the $m$s for which $5^{m}equiv 1mod 16$ holds are $$m=4kquad,quad kin Bbb N$$this is because $5^4=625equiv 1mod 16$ and therefore $$5^{4k}equiv5^{4k-4}equivcdots equiv 5^{4}equiv 1mod 16$$
Conclusion
All $n>5$s for which $5^n$ ends up with $3125$ can be found from $$n=4k+5quad,quad kin Bbb N$$ and the smallest such $n$ is 9.
$endgroup$
add a comment |
$begingroup$
So, we are looking for all $n>5$ for which $5^nequiv 3125=5^5mod 10000$.
Note that the following equivalence holds for $n>5$:$$5^nequiv 5^5mod 10000\iff \5^{n-4}equiv 5mod 16\iff\5^{n-5}equiv 1mod 16$$Define $mtriangleq n-5ge 1$. Then all the $m$s for which $5^{m}equiv 1mod 16$ holds are $$m=4kquad,quad kin Bbb N$$this is because $5^4=625equiv 1mod 16$ and therefore $$5^{4k}equiv5^{4k-4}equivcdots equiv 5^{4}equiv 1mod 16$$
Conclusion
All $n>5$s for which $5^n$ ends up with $3125$ can be found from $$n=4k+5quad,quad kin Bbb N$$ and the smallest such $n$ is 9.
$endgroup$
add a comment |
$begingroup$
So, we are looking for all $n>5$ for which $5^nequiv 3125=5^5mod 10000$.
Note that the following equivalence holds for $n>5$:$$5^nequiv 5^5mod 10000\iff \5^{n-4}equiv 5mod 16\iff\5^{n-5}equiv 1mod 16$$Define $mtriangleq n-5ge 1$. Then all the $m$s for which $5^{m}equiv 1mod 16$ holds are $$m=4kquad,quad kin Bbb N$$this is because $5^4=625equiv 1mod 16$ and therefore $$5^{4k}equiv5^{4k-4}equivcdots equiv 5^{4}equiv 1mod 16$$
Conclusion
All $n>5$s for which $5^n$ ends up with $3125$ can be found from $$n=4k+5quad,quad kin Bbb N$$ and the smallest such $n$ is 9.
$endgroup$
So, we are looking for all $n>5$ for which $5^nequiv 3125=5^5mod 10000$.
Note that the following equivalence holds for $n>5$:$$5^nequiv 5^5mod 10000\iff \5^{n-4}equiv 5mod 16\iff\5^{n-5}equiv 1mod 16$$Define $mtriangleq n-5ge 1$. Then all the $m$s for which $5^{m}equiv 1mod 16$ holds are $$m=4kquad,quad kin Bbb N$$this is because $5^4=625equiv 1mod 16$ and therefore $$5^{4k}equiv5^{4k-4}equivcdots equiv 5^{4}equiv 1mod 16$$
Conclusion
All $n>5$s for which $5^n$ ends up with $3125$ can be found from $$n=4k+5quad,quad kin Bbb N$$ and the smallest such $n$ is 9.
answered Mar 20 at 20:17
Mostafa AyazMostafa Ayaz
17.8k31039
17.8k31039
add a comment |
add a comment |
$begingroup$
Hint: $5^n equiv 5^5 mod 10^4$ if and only if $5^n equiv 5^5 mod 2^4$. What is the multiplicative order of $5$ mod $16$?
$endgroup$
add a comment |
$begingroup$
Hint: $5^n equiv 5^5 mod 10^4$ if and only if $5^n equiv 5^5 mod 2^4$. What is the multiplicative order of $5$ mod $16$?
$endgroup$
add a comment |
$begingroup$
Hint: $5^n equiv 5^5 mod 10^4$ if and only if $5^n equiv 5^5 mod 2^4$. What is the multiplicative order of $5$ mod $16$?
$endgroup$
Hint: $5^n equiv 5^5 mod 10^4$ if and only if $5^n equiv 5^5 mod 2^4$. What is the multiplicative order of $5$ mod $16$?
answered Mar 20 at 20:12
Robert IsraelRobert Israel
329k23217470
329k23217470
add a comment |
add a comment |
$begingroup$
Well
$$5^9=1953125$$
so the answer is $9$. In fact
$$5^nequiv 5^{n-4} mod{10^4}$$
For $nge 8$, so any value of $5^{5+4k}$ where $kinmathbb{N}$ has the last four digits $3125$.
$endgroup$
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
add a comment |
$begingroup$
Well
$$5^9=1953125$$
so the answer is $9$. In fact
$$5^nequiv 5^{n-4} mod{10^4}$$
For $nge 8$, so any value of $5^{5+4k}$ where $kinmathbb{N}$ has the last four digits $3125$.
$endgroup$
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
add a comment |
$begingroup$
Well
$$5^9=1953125$$
so the answer is $9$. In fact
$$5^nequiv 5^{n-4} mod{10^4}$$
For $nge 8$, so any value of $5^{5+4k}$ where $kinmathbb{N}$ has the last four digits $3125$.
$endgroup$
Well
$$5^9=1953125$$
so the answer is $9$. In fact
$$5^nequiv 5^{n-4} mod{10^4}$$
For $nge 8$, so any value of $5^{5+4k}$ where $kinmathbb{N}$ has the last four digits $3125$.
edited Mar 20 at 20:16
answered Mar 20 at 20:11
Peter ForemanPeter Foreman
4,2421216
4,2421216
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
add a comment |
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
Why not $5^5 = 3125$.
$endgroup$
– fleablood
Mar 20 at 20:20
2
2
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
$begingroup$
The question states that $ngt5$
$endgroup$
– Peter Foreman
Mar 20 at 20:22
add a comment |
$begingroup$
Hint $, 5^{large 5+N}! bmod 10^{large 4} = 5^{large 5}(5^{largecolor{#c00} N}! bmod 2^{large 4}).,$ Now recall $, begin{align} 5, &equiv 1!pmod{! color{#c00}4} \ Rightarrow 5^{largecolor{#c00} 4}!&equiv 1^{largecolor{#c00} 4}!!!! pmod{!color{#c00}4^{large 2}}end{align}$
$endgroup$
add a comment |
$begingroup$
Hint $, 5^{large 5+N}! bmod 10^{large 4} = 5^{large 5}(5^{largecolor{#c00} N}! bmod 2^{large 4}).,$ Now recall $, begin{align} 5, &equiv 1!pmod{! color{#c00}4} \ Rightarrow 5^{largecolor{#c00} 4}!&equiv 1^{largecolor{#c00} 4}!!!! pmod{!color{#c00}4^{large 2}}end{align}$
$endgroup$
add a comment |
$begingroup$
Hint $, 5^{large 5+N}! bmod 10^{large 4} = 5^{large 5}(5^{largecolor{#c00} N}! bmod 2^{large 4}).,$ Now recall $, begin{align} 5, &equiv 1!pmod{! color{#c00}4} \ Rightarrow 5^{largecolor{#c00} 4}!&equiv 1^{largecolor{#c00} 4}!!!! pmod{!color{#c00}4^{large 2}}end{align}$
$endgroup$
Hint $, 5^{large 5+N}! bmod 10^{large 4} = 5^{large 5}(5^{largecolor{#c00} N}! bmod 2^{large 4}).,$ Now recall $, begin{align} 5, &equiv 1!pmod{! color{#c00}4} \ Rightarrow 5^{largecolor{#c00} 4}!&equiv 1^{largecolor{#c00} 4}!!!! pmod{!color{#c00}4^{large 2}}end{align}$
edited Mar 20 at 22:24
answered Mar 20 at 21:34
Bill DubuqueBill Dubuque
213k29195654
213k29195654
add a comment |
add a comment |
Catherine Cooper is a new contributor. Be nice, and check out our Code of Conduct.
Catherine Cooper is a new contributor. Be nice, and check out our Code of Conduct.
Catherine Cooper is a new contributor. Be nice, and check out our Code of Conduct.
Catherine Cooper is a new contributor. Be nice, and check out our Code of Conduct.
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1
$begingroup$
What is your take on this?
$endgroup$
– ADITYA PRAKASH
Mar 20 at 20:01
1
$begingroup$
Why not just list them out and find it?
$endgroup$
– Jair Taylor
Mar 20 at 20:02
4
$begingroup$
Why not just do it? It's not $1$ because $5^1=5$. It's not $2$ because $5^2 = 25$. What's to keep you from just continuing?
$endgroup$
– fleablood
Mar 20 at 20:20
$begingroup$
The answer to "What is the smallest such n>5?" is easy, so you might as well retitle the question "What are all n>5 such that...?"
$endgroup$
– smci
Mar 20 at 23:55